The best way to tell is to simply ask her. Being in a relationship is about more then just being with each other it is about being open and honest without the fear of retaliation. You truly need to question why she feels she must be dishonest with you if she is or why you do not trust her if she isn't.
Though to get to your question there are no point blank she is gay actions. But there are little indicators that can determine if there is a possibility.
1: A attraction to other women (She could display this by looking at lesbian porn or pictures of women) Though this is not a complete indication because some women who are turned on by the site of another womens body truly are not gay but simply fantasies about it)
2:She has had relationships in the past with a women.
3: She enjoys strip bars or has made sexual references in the past about other women.
Unfortunately women are not as openly read in this department as men are. Of course if she acts like a man there is a indication there but not all lesbian or bi women act this way. Obviously there is a reason for you to suspect she is or there would be no reason for this question.
Your problem is you must open up the lines of communication to get to the answer you need. Keep a few things in mind first though. Some women do actively fantasy about being bi or lesbian. This is perfectly normal and doesn't indicate she is a lesbian. She simply has a fantasy. Fantasy are sometimes acted out and other times not simply because this is a fantasy only.
Then there is the chance that she is and doesn't feel comfortable talking to you about it yet. And there is the chance she wants to but is afraid. You must get to the root of what it is and work it out not only for yourself but for her as well.
I would recommend sitting down and talking to her. Let her know what you suspect and that you do not judge her in any way. Explain that you know about fantasy's and simply want to work this out together. Let her know you will support her no matter what she chooses. And do so! If this is not your thing let her know that but let her know that you are willing to still be a part of her life. If it is your thing make sure you are not pushing her because it is what you want. This has to be about her and her working it out with your help.