Let's not forget these guys in most cases aren't the most secretive. They like to brag. What is the point of being a made guy, if nobody knows? This is the mindset of today's mobster imo. Of course there are exceptions, but...

When Joey gets his button, everyone in the neighborhood knows. Their big mouths get them in these situations. Kinda like when non-scum get a job promotion- you're gonna tell your friends, etc.

Combine that with wiretaps and informants, and you usually get a good idea of who is doing what.