Would there be anyhing wrong with selecting eveything and snapping if you've forgotton to turn snappining on and you aren't sure what isn't snapped?
Snapping is view-dependent, so it's best to not try and snap too large areas at once, unless it's a more or less flat part, if that makes sense.
Spikey is right about being view dependent. Alternatively you could setup a shrinkwrap modifier for global snapping in the case of forgetting what is and is not snapped. I've definitely been in that situation more than once.
You can use the demonstration in this video for setting up shrinkwrap.