I've seen this before. When the frets are up, they look like they're nicely crowned like a great fret job. When they're down, you're not going to hear a smooth transition in pitch. Slide along the fretboard and when the fingerboard ends, there's going to be a gap until you hit the top of the fret.
I might be wrong, but that's how I see it.
I should get my fretless out and apply myself to it, it's been a while and I feel bad about not doing much about that axe.