If one has been sitting around for a while, you may need to flash it. Make sure it's making electricity. If not, a quick flash is to plug in an angle grinder, pull the trigger, and give it a spin. All you need is a trickle when the generator is running for the field to pick up and excite itself. If that works, the grinder will start to run, so be ready to let go of the trigger or flip the paddle off.
If the angle grinder doesn't work, turn the generator off. Really. The next part will electrocute you if the generator is running.
Get a battery. Most people use a 12 volt car battery because you have to clip one end of jumper cables to the battery and the other end into a receptacle. If you have 240v out, short the battery across the two hot legs. If you only have 120v., do the hot and neutral. Don't worry. It's AC, remember? The magnetism will figure it out.
If you get all sorts of sparks, you probably did it right. Check all the breakers, because you may have tripped them. Start her up and see if she makes electric.
If you are a person of modest morals and a P. T. Barnum even break sort of guy, you might take advantage of that guy who got the motor running but, "The dad gum thing is dead."