How to check how much force is being applied?
 

I noticed that a lot of parts and components have instructions limiting the force to be applied. Is there a meter that one can use? How do you make sure you are not exceeding the limits?

Torque wrench.

I've got a couple of them: I use one with 3/8 drive for cranks and bottom brackets which I'd probably undertorque if left to my own judgement. I use one with 1/4 drive for high dollar stems and carbon fiber parts which I'd probably overtorque or not tighten evenly if left to my own devices.

Other than that, I pretty much just guess: Little bolt - just snug it down. Big bolt - crank that puppy.

If you don't have a torque wrench, here's a way to educate your guess that works on big bolts. The torque spec for crank bolts is usually around 30 lb/ft. Imagine a 30 pound weight hanging on the end of a foot long wrench handle. Tighten it that much.