<p>She tells you.
You ask, she tells you.
You ask a friend of hers, he or she tells you.
(All highly reliable.)</p>
<p>But since most people don’t have the guts to be in any of these scenarios:
Her friends ask you how you feel about her (moderately reliable).
She walks you home when you haven’t known each other that long (ditto).
She flirts with you (unreliable).
You overanalyze her “Hi, Firebat” and detect hidden affection (highly unreliable).</p>