There are always signs you just have to know what to look for. He may distance himself more, call you less, give you too many details about how his night went (when lying people give more details that usually woudnt be mentioned), seem more distant and emotionally disconnected, wont say i love you or call you pet names on the phone, theres always signs you just have to pay attention.
If you have to ask...Then he is and you know it. Face the problem and break up with him. I mean you are asking for a reason and you don't need problems in your life. Life is hard and dramatic as it is, don't add more to it.
Would be extremely difficult to know and you cannot 'see' what he or she is up too...It is like looking for a needle in a haystack.
that is hard maybe ask him what he has been up to or see who he has added on fb.
it's hard to know if they're far away...