marrying a citizen doesn't make you a citizen. I assume from your question that she has passed on?
You might look in her papers for passports, green cards, papers from the INS etc. Only a citizen can have a US passport. An alien should have a "green card", that is an "alien registration card" as they used to be called, nowadays it is a" Permanent resident card".
If you find any records of voter registration, this would be another sign for citizenship.
I would recommend asking the CIS, formerly known as INS, but their organization is so unbelievably bad that I really can't do it.
