For modern cars they have frame pulling points and with the subscription for the computer it tells you exactly where to place the sensors. In the center of the car there's a spinning laser which the computer decodes the distance from the sensors and from there you get your "3D" alignment. This makes it very easy to do, as long as you have had the proper training to know where to pull efficiently and safely. I went more into light body work and then eventually into the paint shop so I never learned that part of it. But, I shadowed a lot of guys who did frame jobs. With that said, at least on cars, one of the ways to check without the computer (which I witnessed from the better techs) is to use a frame squareness gauge. Basically, if it was a front end collision you could check for square at the rad support points. The FSM had measurements to check for this. For a bike, I have no idea how it's determined exactly.