if married I can tell you as it was done to me:
1- check phone records...there will be an obvious call pattern to one number
2 - phone records again...check text messages...they will all go to the same number
3 - phone records again - late night calls and calls during time when 'SO' 'went to the restroom'
4 - late nights
5 - checking on when you will and won't be home
6 - drinking more - SO will disguise
7- read SO journal if they keep one
8 - check their email
9 - if all else fails ASK THEM FLAT OUT.
If its a boyfriend and you are not married....assume he is and