Knowing what trips the audit wire doesn't mean you should forego legitimate deductions, such as those for large medical expenses. But it does mean you should keep diligent records to back up your numbers in case the IRS targets you. The IRS uses what's called a Discriminant Function System (DIF) that assigns a numeric score to each return to kick out those that are questionable, according to IRS Publication 556, Examination of Returns, Appeal Rights, and Claims for Refund.

"If your tax return is selected from DIF, it has received a high score. This means that there is a high potential for an examination of your return to result in change to your income tax liability," the IRS says. If the computer tags you, it may be because your return contained an item that's often claimed erroneously or misreported, or your numbers deviate substantially from the norm. Uncle Sam has three years from the due date of your return or the day you file it to audit you -- unless you've omitted more than 25 percent of the reportable income on your return, in which case he can take up to six years, said Martin Nissenbaum, national director of personal income tax planning at Ernst & Young. But if you've committed fraud, the sky's the limit. The IRS can take a lifetime to hunt you down. 15 ways to trouble

That said, we've asked tax professionals around the country what items make the IRS perk up. Keep in mind, though, that this list is by no means exhaustive.

1. This year, donated cars are "on the IRS list of things to watch out for," said Bill Fleming, director of personal financial services with Pricewaterhouse Coopers. "The IRS sees all these billboards -- 'donate your car, get a tax deduction' -- and they figure something's going on." Taxpayers should check the car's value using a guide such as the Kelley Blue Book, and then adjust that value based on the condition of the vehicle, such as excess mileage or body damage, said Roger A. Kahan, a Stoughton, Mass.-based CPA. A written appraisal is required for cars valued at more than $5,000.

2. Home-office deductions are a red flag to the IRS, because many people take them erroneously, experts said. "If the home office is for convenience, the IRS doesn't allow it," said George Egan, a Richmond, Calif.-based enrolled agent. For instance, employees who prefer to work at home but could work at the office are not eligible, he said. Those who do the majority of their administrative work from home and who use their home-office space exclusively for that work (that is, the kids don't play there and no one uses it as a bedroom) are eligible for the deduction, experts said.

3. Large charitable contributions may raise an eyebrow. "If you give 8, 9 10 percent of your income, you're likely to be a target," Fleming said. For any amount over $250, "you need a receipt and a thank-you note and the note has to have the magic words on it: No goods or services were received," Fleming said. If you don't have the letter and you're audited, the deduction is no good. The risk of an audit "doesn't mean stop giving. That's not a good response. Instead, keep your records and your thank-you note," Fleming said.

4. Hobby or business? Entrepreneurs who claim losses for 2002 may have their business plan examined. "What the IRS will try to show is, if you've been in business for 3 or 4 years and haven't made money, then it's not a business, it's a hobby," said Doug Stives, a CPA and partner with The Curchin Group in Red Bank, N.J. One IRS test: Are you having fun? Your business plan, including marketing activities, must show you're working. "You're not having fun, you're losing money," Stives said.

5. Claiming capital losses may glean some extra attention, because a lot of people do it incorrectly. "More than ever, I'm getting questions (from clients) in the capital loss area," Stives said. Stock must be sold for a loss to be realized, but some taxpayers don't realize that. "They're taking massive capital losses when they didn't even sell anything." Taxpayers are limited to offsetting capital gains or $3,000 of ordinary income. "If you claim a loss and there's no report from the broker that the stock was sold, you're going to get challenged," Stives said.

6. If you take higher-than-average deductions for, say, medical, travel or entertainment expenses you might get IRS attention. "If something looks really odd -- suppose it's a very significant medical expense because a spouse was in the hospital for the last 6 months of life -- you want to make sure to retain all the documents," said Abe Carnow, a Los Angeles-based CPA. And those who claim entertainment or travel expenses will want to make sure their business justifies that expense. Said Egan: "People want to go to seminars. They'll claim a travel expense but there's really no business value in the seminar."

7. Schedule C always seems to get people, Fleming said. That's especially true if you have a cash business such as a tavern, since the potential to fudge numbers is pretty high. So the IRS will be looking for receipts. If you are in a cash business, it will expect to see deposit slips corresponding to the amount of income you report. With a home business, the lines between personal and business expenses sometimes get blurred. For example, a contractor who is building a house for a customer may opt to use some of the materials for his own home remodeling. In that case, the costs of the materials used on his own home should not be included as part of his business expenses.

8. People who claim the earned income credit may continue to see some extra attention. "The IRS got some heat for focusing on people in lower income brackets, but it's likely they will continue to look at those who claim the EIC," Nissenbaum said. "The EIC is a refundable credit -- it's likely they will continue to focus on that when it looks suspicious or when someone claims a large credit."

9. The IRS receives copies of all your W-2s, 1099s, 1098s, etc. It matches its copies against the ones you attach to your tax return, and ensures that the numbers reported on your return correspond as well. If they don't match or the forms are not included, expect to get a notice from Uncle Sam.

10. Not filing on time inevitably will make your return stick out. So if you're trying to keep a low profile, be sure to file for an extension if you can't get your return in by April 15.

11. If you happen to be getting an exceptionally large refund, don't expect Uncle Sam to freely give up the money. He's going to check and double check to make sure you really deserve all that cash back.

12. Any state and local refunds you may have received must be accounted for on your federal return, or you can expect to be contacted by the IRS.

13. Individuals receiving income from a partnership or S corporation will hear from the IRS if their personal return does not match what the partnership or corporation reported. "The IRS will compare the Schedule K-1 to the tax return and notifying taxpayers if there's a variance," Kahan said. "Last year they sent out thousands of notices to people ... they said they're going to start the program again this year."

14. If you have a history of noncompliance, or someone tips the IRS to your shenanigans, you most likely can expect your return to be examined.

15. Math errors, incorrect or missing Social Security numbers for you or your dependents and the lack of required information inevitably will trigger correspondence from the IRS as well. Remember that even children less than 1 year old require a social security number.

Granted, there are worse things than an IRS inquiry. But it could turn out to be a paperwork headache. So do yourself a favor and get it right the first time. Be scrupulous and attentive to the details and hopefully you'll never have to hear from Uncle Sam.