The function of the trigger group can be checked out of the gun. When you pull the trigger, the rear arm lowers and stays lowered when you release the trigger. When it's in the gun, the rearward traveling bolt has a tab on the bottom of the bolt that hits the small lever in the top front of the trigger group, and pushes it rearward. This releases the sear (rear lever) to go up and catch the bolt (in the open position). Pull the trigger again, and the sear goes down. releasing the bolt to go forward (if it's in the gun) . Push rearward on the little front lever, and the sear pops back up. Testing the function of the safety is obvious, and there really isn't much else to it unless you have broken or bent parts. Soaking in kerosene and blowing it out with an airgun is the easiest way to clean it. A few drops of oil and you're good to go after that.
I've done a few one-on-one sessions for the MG42 at my place. I set up a table and chairs in the garage and go through everything I can think of. When someone brings their gun and all the accessories, I usually manage to kill 3 hours. I had one guy bring 20+ barrels and 5 bolts, and we checked headspace on every combination. We found a few that couldn't work together, but most were good. I even managed to fix his belt loader that had been dropped. In short, a good time was had by both of us. I live in the country and can safely shoot in my side yard. The neighbors don't care, and having been a deputy sheriff for 5 years a while back doesn't hurt when it comes to any complaints the sheriff gets about machinegun fire.
The manuals I've translated have educated me quite a bit about the gun. Several years as a car mechanic doesn't hurt either for diagnosing the mechanical problems.
I've also had people bring their broken guns to me at knob creek. I may not have the tools, but I usually manage to tell them the problem and how to fix it.