Taxpayers have to be more careful than ever about cross-checking the information on their tax return. The IRS computers are getting very picky. Small deviations in tax reporting that used to go unnoticed are now being challenged. Among them:

*If you got a state or local tax refund last year, it will be reported to the IRS (on Form 1099-G) as part of your taxable income for 1985. But if you took the standard deduction last year, you won't owe a tax on this money. So put a note on your tax return, explaining why you have not included the refund as part of your income. Otherwise, the IRS computers will put you on their "wanted" list.

If you itemized deductions last year, however, you will owe a tax on your state or local tax refund. Enter the amount on the 1099-G on line 10 of the 1040 tax return. When you deduct this year's state tax payment, do it on Schedule A, line 6.

*If your company gives you a car, and some of your driving is for personal reasons (like commuting to work), you have received a taxable benefit. Part of the cost of that car will be added to your salary and reported to the IRS on Form W-2. This could raise your taxable income by $1,000 or more.

Your company was supposed to have withheld taxes on this form of income or else posted a notice that no taxes were being withheld. But because of widespread confusion about the new regulations, many employes didn't learn about this auto-related income until they got their W-2 forms. Unexpectedly, they are having to pay an extra tax.

If you use your own car for business, you'll have to answer a long list of detailed questions -- and getting them wrong could trigger an audit.

*You'll have some explaining to do if you mail your Individual Retirement Account contribution on April 15.

Legally, it's deductible on your 1985 return as long as the postmark on the envelope shows that you met the deadline. But your IRA trustee -- for example, your bank -- will show that contribution as having been received after April 15, and will report that to the IRS.

Result: You'll get a computer notice, claiming that you weren't entitled to an IRA deduction for 1985, and it might take hours of work to prove that you mailed it on time. Moral: If you're going to mail an IRA contribution, do it early enough for the trustee to receive it before April 15.

*You have until April 15 to contribute to last year's IRA. But you can stretch out your payments to a previously established Keogh plan until Oct. 15 and still deduct them on your 1985 tax return, as long as you keep filing for extensions.

It's too late to start a new Keogh plan; the deadline was Dec. 31. But Peter Elinsky of Peat Marwick reminds you that you still can start a SEP (Simplified Employe Pension plan), and make the equal of a Keogh contribution, right up until April 15. If you have employes, the SEP has to cover them. But you also can start this plan if you're self-employed.

*Ellen Murphy of the IRS says that one of the biggest reasons for delayed refunds is that the taxpayer's name and Social Security number don't match. If a woman marries and changes her name, she should notify Social Security immediately; she also should include her former name on her tax return and note that she married during the year.

*Because of computer snafus and frustrated personnel, the IRS made hash of a lot of tax returns last year. Some were shredded; some were lost; some were posted incorrectly; some haven't been posted yet.

If you're self-employed and yours was one of the mishandled returns (which could have happened without your knowledge), your 1984 earnings might not have been posted to your Social Security account. If you let the error stand, it could reduce your ultimate Social Security check.

Such an accident isn't supposed to have happened to anyone who's an employe. But check your earnings record through your local Social Security office, just in case.

*The earned-income credit has increased. You now qualify if you earn less than $11,000 and make a home for a child. Last year the ceiling was $10,000.

The credit is figured on a sliding scale -- rising gradually from $3 (for people earning $50 or less) to a maximum of $550 for incomes between $5,000 to $6,500, then declining again. If you earn $10,500, you'll get $64.

If your credit exceeds the amount of tax withheld from your earnings, you can file for a refund. That is, if you can figure it all out. The earned-income credit is so troublesome to calculate that a lot of people who could use the help just pass this tax break by.