Hi, i would like to just ask if how would you know someone from a long distance relationship is two timing someone?

dont get me wrong, im not involve in a relationship, just wanted to ask about the matter.
Given a situation, a filipina in the PI is having a "relationship" or "mutual understanding" with someone abroad, lets say from the UK.

Forgive me for saying this but usually Men (rather than filipina women) have the tendency to stray. And in a relationship, honesty is the one key that is very important. My question would be how would "you" know if the guy is seeing someone other than yourself, even if he say he isnt? (assuming he is seeing)

Is there some indications or perhaps tips that you might shed in this matter?

Answers/opinions are very much appreciated.