W124 with M103 will not crank
 

Twice in the past few days, when I attempted to start the engine, I turned the key to start but it did not crank. Rotating the key between switch off and start three to five times did get it to crank. How can I check to find if this is an ignition switch problem or a starter solenoid problem.

Check your battery. If your battery checks out, get under the car and tap the starter with a hammer a few times.

That could work
 

But of course the problem is quite intermittent. If it would decide not to crank when I had it up on ramps, I would certainly try that. But of course, being a random failure makes that approach difficult. The starter is on the drivers side and not to far back. I just might be able to tap it with a hammer from the top. I will carry a small hammer in the trunk to see if that could work.