A complete and in depth investigation is key. Even if everything looks ok at first glance, probe, ask questions and always listen to your gut. Chances are, if something does not feel right, something is indeed very wrong.
In the following example everything would appear ok at first glance. No one would know. Taking off the cover you would see wire connectors, no double taps, one neutral and ground per hole, 20 amp breaker on appropriate #12. All good so far, but probing further like opening the panel in the closet or asking “why am I seeing new wire in a 30+ year old building?” will save a life.
This is the PERFECT reason why a home inspection is essential. My apologies for the length, but an excellent educational example of improper and dangerous electrical IMO: