8 Answers

You just don't until she or someone else tells you otherwise. Be careful with your suspicion, it can drive you away from a great relationship.

When you positivity, absolutely catch her red handed...

Get'er  down  and  smell her   out  see if   she's  got any  unusual   male  oders  on or around  her person,  if you detect a strange  oder   that  could put her under suspision.   Good luck  we're all counting on ya!

She runs you over with her car on the way to see her other boyfriend.

SHE TOLD ME TO TELL YOU TO GET OFF HER COMPUTER.

I forgot to add this in my last answer,  so   be sure to  do the  sniff down in a  nonchalant way ,  sorta act like  your  gonna nibble on her ear   then get close and  casually   get  that nose up close  and  get a  good whiff   up both   nasal passages    to make sure you got   adaquate  amount  of  possible male pheromones  up the ole  snoot.  if  she becomes suspisous  tell her  its  just alllergies do this   several places   in or around  the  body   see if you can pick up the  scent of a man.   As  Always  GOOD LUCK WE'RE ALL COUNTING ON YA!

I'll take your question seriously, and refer you to Village Health website.  They had an article recently about the "warning signs" of cheating.  It may be helpful to you. 
It is my sincere hope that your fears are unwarranted.  It is so very hurtful when that happens.

She would be wanting to spend far less time with you cause shes obviously got other things to do with her time.