How can one check if he is a Cohen? My husbands’ aunt told him that she thinks he is a Cohen. His father is not sure. Its hard to trace their family ancestry, they are from Odessa Ukraine.

This is not a simple question. Generally you will need some existing knowledge or new proof of being a Cohen, i.e. family member that is known to be a Cohen or a family tradition. Since there is no clear evidence in this case' it is difficult to reach a positive conclusion only through a vague comment of the aunty, it should be investigated what her thoughts are based on etc. (See Responsa Yachel Yisrael 30) Consult your local rabbinic authority for a practical follow-up.