fish on somebody (gangster slang) -- meaning?

Question

Cops undercover doing some illegal street buys (from the movie RoboCop (2014)):

— Where the hell you get a gun like that, Jerry?
— I can get you two whole crates full, man.
— Two crates? No disrespect, J, but how a clown like you get so much product, man?
— What's it matter where it comes from?
— Why it matters, we don't want no dirty guns, Jerry. These guns got bodies on 'em, Jerry?
— Yo, look, they're clean, all right? You gonna buy this shit or not?
— Why you acting so nervous, man?
— You fishin' on us, Jerry?
— What'd you say?
— You think he fishin', D?
— I think he's fishin' on us.
— Yo, what the hell are they talkin' about, man?
— Hey, let me see your phone.

I searched online and couldn't find a thing about this expression.

Answer 1

Think of normal fishing. You put some bait on the hook and put it down into the lake/ocean/sea. You are tricking the fish into eating the bait, so that you can catch it.

So what fishin' means in the usage here, is that he is wondering if Jerry is really a cop, trying to bait them into committing a crime (buying illegal guns).

So once they say they'll buy the guns/drugs/whatever then the cop can arrest them for doing something illegal.

Not saying that Jerry is really a cop (haven't seen the movie), just seems the other guy was pondering whether or not Jerry was.

Answer 2

Before I got to Vietnam, I and others in my interpreter-translator class were warned that the Old Guys might fish us, that is, pull our legs for a while, trick us or have us new guys do something that turns out to be pointless. One example of fishing a "nug" (new guy) was sending him to get a copy of the "unlisted call-sign book." Someone would send the new guys somewhere to get this non-existent book, that section would send him to another and so on.