I have only ever been able to tell when it was someone I did not want to have a crush on me, someone I was definitely not interested in. If it was a woman I was interested in, or even someone not on my radar but that I probably would’ve seen how things went had I known she was interested in me, I have not figured it out until years later, if ever. My now-wife had to come right out and tell me she was interested. I would not be surprised to find out there were people I’d completely missed hints from decades ago.