lets take a step back here anon and try to remove your bias from the situation as much as possible
do you have any actual evidence or anything that would make you think that other than "she has lots of guy friends, one of which is her personal trainer"?
be honest, is this just insecurity?
im not saying she is or is not cheating on you i just want you to actually think about this objectively for a bit before you do anything drastic, especially because this is always such an inherently difficult topic to bring up with your SO
how long have you guys been dating?
do you guys live together?
has she been behaving strangely around you?
have you been behaving strangely around her that would make you think shes behaving strangely around her? those are two different questions, think about that first before you answer either of them
sometimes there are signs and sometimes there are not. sometimes people noticeably change their behaviour, and sometimes they are good at hiding it