Generally, if you receive income other than social security you will have to file a tax return. Part of your social security maybe taxable if your income is greater than the base set for 2009.

$32,000 for married couples filing jointly.

$25,000 for single, head of household, qualifying widow/widower with a dependent child, or married individuals filing separately who did not live with their spouses at any time during the year.

There are other considerations that may come into plan, I suggest you check out the IRS web site for more details, there is a special section just for seniors.
http://www.irs.gov/individuals/retirees/index.html

Tax time is just around the corner again, so it's wise to keep note of when you need to file in retirement. Generally, it's smart to file if you have any income besides Social Security. Disbursements from retirement accounts, pensions, interest on investments, and other types of income are all considered taxable. This makes you subject to paying income tax.

In your case, the retirement check is probably a pension. If so, this is taxable. Check the stub or statement included with your pension check to see if taxes are deducted in advance. Sometimes they are, in which case you won't necessarily need to file. In other cases, there is no deduction, and it's likely you will have to file and pay the income tax.

Don't forget to follow up with state income taxes too, unless you're lucky enough to be in a state without one. Many types of retirement income miss taking out these taxes off the top, both from the Federal and state level. Even if you owe no tax, it doesn't hurt to file. You may qualify for credits that differ from year to year, or receive a surprising refund. It also gives you evidence to cite in case you receive a notice from the IRS wanting money after pulling an incorrect audit on you.