All Individuals that worked in United States, including those that worked seasonably, such as students with J1 visas, should file income tax returns unless their gross yearly income is less than the minimum amount of income that IRS requires individuals to file a return. This minimum amount changes yearly and it's posted in IRS publications. If the source of income is wages and the amount is less than the personal exemption amount (see Publication 501), then the individual is not required to file.
According to studies a lot of nonresidents overpay their taxes, since most of them do not file a return or file it incorrectly. When the employer withholds tax from the pay it doesn’t mean that it is the proper amount. Therefore even in a situation when not required to file a return, I recommend preparing the return or at least estimating it to check if there is any refund due. To find out the amount of a refund use one of the tax estimator tools available on the Internet or prepare an actual tax return. Using the estimator it’s quicker but it's just an estimate. Preparing a tax return is more accurate and once complete it’s ready to be filed. The accredited online tax preparation services such as Turbo Tax or H&R Block let you prepare tax for free with no obligation. Pay only when you want to file, and in some cases filing is also free.
If you worked seasonably in United States and had taxes deducted from your pay then you can file an income tax return to get the refund of taxes that you paid through your employer. Generally most of the deducted tax is refunded when your income is less than a standard deduction. And if you qualify for some credits, such as Earned Income Credit, then your refund may be larger than what you paid in taxes. However most credits are not allowed for non-residents. Individuals that worked seasonably are considered non-residents and need to file returns on forms 1040NR or 1040NR-EZ depending on what they need to claim on the return.
If you worked legally and on a payroll, your employer will send you a W-2 form, which usually arrives in the mail during months of January or February. You may also pick it up yourself if you arrange that with your employer, and given that you didn’t leave the country. As W-2 form is the most common form for reporting income, your employer may report your income on form 1099-MISC, but that’s more rare. Besides this, you may also report your cash income as a self-employed on a Schedule C, but you would usually not receive refund unless you have prepaid tax during the year.
The bottom line is that you obviously want to file tax return if you have any refund. I also recommend filing, regardless if you need to pay or getting a refund, when you’re thinking or are in the process of becoming legalized by any means, as tax returns are some of the most important documents that immigration is reviewing and considering. However in other situations, if your income for seasonal work places you in the bracket where IRS requires you to file tax return and you’re not getting a refund, no one will come after you if you don’t file a return.