Have you have filed your 2015 taxes yet? If you are separated or divorced, there are a number of rules regarding your taxes. For IRS purposes, married means married. If you are married, you CANNOT file as single. Your only possible options are Married Filing Jointly, Married Filing Separately or (perhaps) Head of Household. You don't have to be in separate household to file as married filing separately, but be aware that there are a number of deductions and credits that do not apply if that is your filing status. Often, married filing separate is not the most advantageous way to file, although an experienced tax professional will be able to help you determine that. If you are eligible to file as Head of Household, that is more generous, however you must be unmarried, or separated for more than the last six months of the year, pay more than half the costs of the household AND have a qualifying child. And, yes, the IRS has definitions for all those elements as well. Often, my clients will cooperate in filing jointly for as long as they are eligible, however, you are then responsible for the accuracy of everything on that return. If you have reason to doubt your spouse's truthfulness, you may need to file separately just to protect yourself.
IRS Publication 504 has the rules for divorced and separated couples, but I recommend talking with an experienced tax professional and save yourself the headaches.