How can I tell if the girl I'm with loves someone else?

as a guy who has dealt with this its not hard to see, if she talks to a certain guy alot or if they dated prior that could tell. also if when you talk to her she gets defensive when a certain person is around. a HUGE way to tell is iff she starts dressing better or is wearing more makeup ect. also if shes avoiding you at all wanting to hang out with her friends more and doesnt explain why i hope this helps you find a heart breaker