Easy. I am a girl so i can help you with this one.
1. If the girl giggles with you or around you.
2. If you catch her looking at you or staring.
3. If when you talk to her she gazes in your eyes.
4....
It's obvious if someone is flirting with your boyfriend. The question should be, "Does your boyfriend care?" The answer to THAT question determines how you should handle the situation. ;)