So what would be concrete evidence if someone was on probation and the probation officer was trying to revoke them? Say if someone said they were drinking? What would be the concrete evidence that would be needed to get them revoked?

What would be concrete evidence of drinking? A failed drug test or an arrest for DUI or DWI would be concrete evidence.

The probation officer sends the field agent to talk to people at a bar, or several other witness, they find photos of you at party drinking

With someone telling them, they may decide to do more random testing, and perhaps have someone do a search of your home to see if they find something