Listen, I know it is really bothering you to question if your loved one is being unfaithful.  I have been cheated on by the same guy over 20 times. I am no longer with him thank God!  The subtle hits he gave me were- Blamed you for cheating, over protective(more than normal).  Not calling for a long period of time or disapering and can not be contacted.  Doesn't cll you back when she says she promises she will (and for no reason doesnt call back) Super lame excuses. 

  But the three that told all was when they

#1 studering when you are asking them about this situation.

#2 Can't look you in the eye or constatly looking away or down.

#3 Repeats the question you just asked or answering a question with a question.

   Yet sometimes a person may be going through masive changes or grieving for something or someone.

If you know they are going through a hard time, try not to jump to conclustions.  Use what you know about the person to help you decside, follow your heart and ask your self...Is it worth it?