Green’s Dictionary of Slang

peep v.3

[peep n.1 (1)/SE peep, to look furtively]

1. [1950s+] (US black) to scrutinise, to uncover something that was meant to be kept secret by checking.

2. [1960s+] (also peep out) to put someone or something under surveillance; in weak use, to watch, to look at.

3. [1990s+] to pay close attention, to listen to what somebody is saying.

In phrases

peep things out (v.)

[1990s+] (US black) to see what is going on.