Timing is important here, some will cum quickly but from my experience most require a solid 15-20 minutes of head before they really cum. At a minimum some might get off in 3-5 minutes. At 5 minutes only a fraction will have cum (and most guys think this is enough because we can cum that quickly), but after 20 minutes the orgasam rate is closer to 90%.

Best advice is to read the book She Comes First it's literally a game changer. I thought I was good at eating out before (one girl even compared me to her lesbian friend lol) but after reading this book my GF actually noticed a difference. As in "holy shit, what are you doing down there that feels fucking amazi-OH GOD!"

As to how to tell if she actually came vs faking? The one physiological sign they can't hide is the skin flushing in the collar / neck / breast area. It's super noticeable, watch a few porn videos and you'll notice when an actress actually cums vs when they fake. Everything else can be faked, but this can't. Even on darker girls it's noticeable.

And in the words of Louis CK: If you fucked her properly she won't tell you how good it was or that she wants to cuddle. No, she's doing this because she's actually not fully satisfied and is trying to seduce you again for round 2. If you DID fuck her right then she won't be able to form a compete sentence for a few minutes.