Casually tell her about the other girl. She might not openly show that you are jealous, but if she's experiencing feelings for you, it will be frustrating and most likely her mood will deteriorate. Most likely she will resent you, and you have to beg her forgiveness, but you will be assured of her sincerity.
Or another option is to ask your friend clearly flirting with her. Loving a woman is not interested in other men. If she respects you and she cares for you, she will not respond to his advances, and possibly even in the correct form would stop all communications with him.
Ask her for help. No matter what kind of service it would be: rearrangement of furniture or repairs in the apartment, buying new jeans or products - most importantly, that she gladly agreed to help. After all, with a loved one you want to spend as much time as possible.
An important symptom of love is that in quarrels she tries to compromise, looking for solutions to problems. If she is guilty in disorder, her irreproachable be the first to ask your forgiveness. The desire to hurry to finish a quarrel and a compromise suggests that she certainly cherishes you. A woman is ready to melt into the man she can not long be angry and frustrated.
Extreme way - the collapse for some time out of her life. If you mean so much to her, she will freak out, try using friends to contact you, cut off your phone and cast aside SMS. If you for her only entertainment, the answer will hear ringing silence.
You can think of a lot of different tricks, but the best way to calmly talk to a girl, try to dot the "i". Not always what we see is real. Try asking her the motives of her behavior, to listen to her point of view, and then draw conclusions.