I’ve had some luck with certain types of machines. One I came across recently had a ‘feature’ where you could shake the handle from stop to stop before putting the quarter in and sour tarts poured out. They were terrible but, hey, Free!
Another type you can check for: The handle doesn’t have a spring return. This might be hard to explain…this type of machine’s handle doesn’t have a springy feel to it. THis type does not return to horizontial if you give the handle a little turn prior to feeding a quarter. These aren’t common but M&M’s might as well be free.
In general, I’ve had varied luck just shaking the handle. Place quarter, turn handle slowly, listen for items falling. As soon as you hear ‘action’, carefully turn handle from side to side but not so much that it clicks to the next index point. It may or may not work.