If you are in a relationship with a man and you think they are doing this...?...?

I found on his credit card bill where he is paying on a yahoo personals account. I didnt tell him I seen the statement where he paid this bill I just told him that I knew he was a member,he said he used to be but is no longer,the bill is current.How would I go about maybe finding him on yahoo personals, I dont belong to yahoo personals,Ha,go figure ,maybe i'm not in of much as a relationship as I thought I was with this man!!!