Happens more times than I can count, and always in a work setting.

1) After a good convo, guy says "you're cool."
2) Guy starts coming to my cube/ office to "talk" or "catch up."
3) Guy starts offering to do me work-related favors.
4) Guy says some variation of "You're so awesome. Why are you single?"
5) Guy getting more playful/borderline inappropriate w/ touching, or starts hugging me for too long.
6) Guy comes to my cube/ office and says nothing. When I don't do anything, he leaves.
7) Guy either 1) Admits feelings or 2) Gives a platonic "what if?"
8) I listen without judgement, saying something like "I accept what you said, I'm so glad we can remain professional colleagues" or in the platonic situation, tell him what I would do in a way that shoots him down.

Now, how he will take it varies in a few ways.
1) Most often, he'll accept the rejection and continue to be cordial, but more appropriately so.
2) He'll become distant in a weird and obvious way or
3) He'll quit/ transfer jobs (this has only happened in retail).