I'm due to file for citizenship, but I was unemployed last year, and so not filing taxes. Does it compromise my illegibility?

Your employment status is not a problem. Check to see if you are still obligated to file a return, if so, file, and you will be fine for natz. They like to see 3 years of filings and that you owe no taxes.

How do I find out if I have to file? Any idea? Thank you.

Check with a CPA or any reputable tax preparer. If you don't meet the minimum income, you have no obligation to file. But you should file a zero'd out return to show you filed.