You may be able to get relief from tax on a joint return if you qualify as an innocent or injured spouse. There must be an understatement of tax due to "erroneous items" that were the responsibility of your spouse or former spouse. These could include hiding a winning lottery ticket or embezzled money to support a drug habit or extra lover on the side. The income still belongs on a tax return and if you signed it, you are liable for everything. Your part of a large refund check may also end up going to pay your spouse's back child support making you "injured" in the eyes of IRS.
If you didn't know about the income and have no reason to know about it or the IRS can be convinced it would be unfair to make you pay, you can get innocent spouse relief and won't have to pay. If you don't live in a community property state, you can file for your part of a refund if it was offset for your spouse's debts.
If you have doubts about a return that is given to you to sign, you shouldn't sign it. You have the right to file a separate return.
An application for innocent spouse relief is based on a subjective evaluation of facts. I can tell you if you have a case and present it in the best possible light.
Michael S Cash Tax Resolution & Assistance 426 14th St, Suite 111, Modesto, CA 95354 (209) 484-5789