Electricians can look for inferior materials and techniques by checking the fuse box or circuit breaker box, and by sampling some outlets and switchs. They can do circuit audits to make sure that circuits are not overloaded relative to the breakers in the breaker box.

They can check wiring for power leaking to the ground or neutral lines, crossed ground & neutral lines, and excessive resistance in circuits. Actually there are pretty inexpensive circuit checkers that can do some of that for you - you just plug them into an outlet and read the lights.

I'm by no means an expert, but having run power to - and installed the conduit, circuit breaker box, ground, and wiring for my horse barn taught me a lot about the topic, and helped me gain a very healthy respect for what electricians need to know. I know only a fraction of what a good electrcian knows.