Star Trek: Deep Space Nine, Season 4

Joseph:
I've got a dermal regenerator under the...(Joseph notices Sisko staring at the blood, and realises what he's wondering about) Benjamin Lafayette Sisko! What the hell has gotten into your head? You actually thought I was one of them, didn't you?

Sisko (desperate, upset):
I don't know. I wasn't sure.

Joseph:
This business has got you so twisted around you can't think straight. You're seeing shape-shifters everywhere. Maybe you ought to think about something for a minute. If I was a smart shape-shifter, a really good one, the first thing I would do would be to grab some poor soul off the street, absorb every ounce of his blood, and let it out on cue whenever someone like you tried to test me. Don't you see? There isn't a test that's been created a smart man can't find his way around. You aren't going to catch shape-shifters using some gadget.