See if she is focusing all the time on a guy, but not flirting or talking with him. Watch her eyes. See if she deliberately gets close to you while she is watching a particular guy at the same time.
Don't get drunk while with her so you can be alert to these signals.
Also, if you suspect this, drop hints about it to her girl friends. They won't tell you, but you can tell by THEIR eyes and expressions when you ask about the girl and another guy, if something like what you suspect is going on.
does she ignore you unless there are people around?
Does she maybe post flirtatious posts on your FB but doesn't do or say much in private?
If you go out are there typically guys that you think she may like who happen to be at the same place?
Otherwise it's kind of hard to know. Essentially all you can assume though is if she treats you differently in private (cold, unaffectionate, standoff-ish) then she does in public (flirtatious, affectionate, etc)
You won't know for sure until she lets you find out. Allot of girls (I don't describe them as women on purpose) like to lead you on real strong just so they can appear to be about to be taken off of the market.
I had a 32 year old girl pull that on me because she wanted a real merican not so alpha boy co worker who still lived with his mother. She led on real strong and seemed to get right on me if I failed to call her at work claiming she just had to talk to me. Her ploy worked, all of the sudden she had nothing to say to me again. That's how you know.
Basically, her affection towards you will be a show. Her attention in you will exit left when the crowd leaves. In other words, there will be a noticeable distinction between the attention that she gives to you in private versus the attention and affection that she gives to you in public.