How do I identify bisexuality in a female?
Better ask them straight. There is really no other reliable way to determine that, although seeing her engaging in romantic or sexual acts with both men and women is indicative without being conclusive.
However, unless you are a lesbian or bi woman who is interested in being with this hypothetical possible bisexual woman who you know is interested in men, you are more likely to get a rude or snide response than an answer to your question. Because, quite frankly, the sexual orientations of others are none of your business, unless you are interested in romantic connections with them.