(IAAC) limiting magnitude

Hello everyone,
publication that with a known limiting magnitude will tell you if an
object you are trying to observe will be bright enough to see? For
example, if you have a limiting magnitude of 6, is there a way to
determine if you can see a magnitude 12 object? I think someone once
told me of a book that had information in it that would let you
determine that. Or, is there some type of formula you can work with that
will let you know?
I know experience is the best teacher. But, right now, I feel like I'm
wasting a lot of time looking for things that are just too dim for my
sky conditions. The problem is that I don't know if the objects are too
dim or if I'm just not pointing the telescope at the exact right spot.
Can anyone help me on this one?