There are a lot of things that had to be in consideration when to do a boost leak.

1. If you running a draw-thru setup then you will feel it (car will misfire at times when the leak is big), and you will see it (A/F ratios will be way off target.

2. If you are in a blow-thru setup then you will not fell it since the leaking air hasn't been metered yet, but you will be able to see it by not achieving the desired boost level. But sometimes the leak is a cracked VAC line so you will not even see it.

3. There are numerous problems that have the same symptoms as a boost leak.

Most of the time after a asking the costumer a couple of questions I know if a boost leak test is a must, or if a visual inspection leads first.

Rafa