How not to get shocked: unplug it. Then you are safe. Well, for an EM pinball machine anyway. Other things might have capacitors that hold a charge that can be dangerous for some time after it is unplugged, even fatal (like the old TVs and crts).
Standard safety protocol is to always turn off the power before poking around inside a machine. Yet, techs poke around inside with the power on all the time. Sometimes you just have to in order to get at a problem. If you are going that route, then the more knowledge you have, the better.
There are the standard rules of thumb. One says that you only ever use one hand while poking around in there. That way if you touch something live the current will go down your arm to your leg to the ground. If you have two hands in there it could make a path directly across through your heart. That would not be good.
It seems obvious, but it is best to minimize the use of any tools made of metal while inside the live machine. That shit conducts. Plastic or wood poker-arounder tools are better.
It is best to have a good knowledge of every single thing that is inside the thing you are working on. For an EM machine, you should know exactly what is going on at every point you are getting near. There is live 110 VAC coming in, and that is found at the transformer and fuse block area, and in some machines in the coin door. That’s an area to use the highest caution around. Everything else is around 50 VAC except the lights which are 6 VAC.
There is no DC in most EM machines. But some do have it. But really AC or DC are both dangerous. And, it isn’t the voltage that kills you, it’s the current. Standing barefoot on a wet floor while working on a live machine is not recommended.
Will it kill you if you accidentally touch a live voltage? Probably not. But it could. It will however let you know it. You’ll probably suffer more pain as a result of your fast reaction to the voltage as you jerk your hand away, bumping into whatever sharp mechanical contrivances are nearby or possibly even knocking down the playfield on top of your head. Like a hot stove, you learn quick to keep your distance.
Long ago I was an electronic tech and we did live troubleshooting on all sorts of equipment. I got shocked a number of times. Usually it was nothing bad, just a quick one that doesn’t do much of anything. But I did take a 230 VAC hit once, that one was pretty hefty. It was a shocker.
The bottom line is, if you have to ask, if you are not very sure of what you are doing, then cut the power. If you know what you are doing and maintain caution and vigilance at all times, you probably will be ok to work with the power on. But you shouldn’t. But we do it all the time. But it’s not recommended.
As a final note, it’s probably best to not work on a live machine while drinking or enjoying the recreational drug of your choice. 
