How do you be sure tensioner and crank are in line?

How do you be sure tensioner and crank are in line?
By site alone, one should be able to see alignment, however is this serpentine or v-belt... V-belts can be aligned with a square and are tolerant being out of perfect alignment, whereas serpentine belts are much less tolerant also with the addition of pulleys riding on the back side of the belt without grooves, they are more apt to flip off. Many factors can cause belt alignment issues, crank bearing thrust wear, accessories (p/s, alt, air pump) worn or out of align, tensioner / idler wear, etc, but if you talking stock engine with factory accessories, brackets, etc - give it a look, if your flipping belt/s, take them off, check for end play on pulleys, idlers, etc, serpentine tensioners and idlers are a replacement items available at most parts outlets...