BG's method works for at rest but you really need to check it at full draw as well. I hold the bow out as if I was going to draw it and eyeball/align the string with the cam grove so it looks like the string is going parallel with the cam then I look where the string crosses in reference to the rest prongs. Then I draw the bow and do the same thing lining the string up with the top cam and looking at where it crosses in reference to the rest. If everything is in oreder and your bow is at least very close to centershot your string should always cross the center of the rest looking at it from behind. If there is cam lean, like on my BowTech, the string should cross the inside of the rest at static position and equally to the outside of the rest at full draw.

The easier/better way is to have a machine that allows you to draw the bow and have it jigged up while you check those reference points with a lazer but we do what we can...