It's been eons since this scenario would apply to me, but I used to rely on dropping social cues, such as a glance-one-second-too-long, or mirroring body language, etc., to gauge interest. Just enough for the person to wonder, "is she interested?", but not long enough to be sure. That gave the other person a chance to pick up on it and make the first move, or to shrug it off and forget it.
Looking back, I am not sure how successfully subtle I was, but as a woman, I felt more comfortable in providing opportunities for the other person to take initiative.
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