Yeah, I just hold down a string at the 1st and 15th frets and tap it against the 7th or 8th just to make sure there's some clearance. As Wez says, a gap about the thickness of a piece of paper.
It depends on your playing style too. If you play mostly rhythm you might want a bit more relief - then you can still have a low enough action around frets 1-5, but the strings have loads of room to move in the middle of the neck without rattling against the frets.
One of my Strats was set up like that - it looked like the previous owner had spent all his time playing root-position chords. Then I came along doing my hackneyed E blues-scale noodling around the 12th fret, and the strings felt like they were a mile off the fingerboard.
If the action is low at the upper frets but feels high in the middle of the neck (like my Strat), you probably have too much relief - the neck has a forward bow. And conversely, if you can't get a low action at the upper frets because the strings are buzzing on the middle frets, you might have a bit of back-bow.
If you do end up adjusting the truss rod, only give it a quarter of a turn (or less) at a time. The neck takes a while to settle itself.