Most common crystals are spec'd at 100 ppm accuracy. The result is that if you get two clocks, both at the worst case error, each in a different direction, you can get as much as 720 ms (that's almost 3/4 second) of skew between them in an hour. In practice, you'll never find two crystals that are that far off from each other. Typically, you'll see less than 20 ppm error between two of the same model of crystal. You can buy 10 ppm crystals, but it's not common for those to be used in audio equipment. Most high spec crystals are used in radio communication equipment.
Anyway, I'm lucky to have two UA-5's that produce timing skews, relative to each other, of less than about 2 or 3 ms per hour, so I can just drop one waveform on top of each other. Back when I used my JB3's alone, I would get more like 15 ms per hour between them, which meant that I had to align them track by track.
Now here's where I was really trying to go with this: When you line up two sources and haven't used a tone generator like Chris was suggesting, look for drums by themselves between tracks in the middle of the show. It's pretty easy to line those up. If you have to resort to lining up track by track instead of a whole show, then you might want to consider lining the tracks up in the middle of the track. That way, your tracks end up with less average skew than if you lined them up at the first of the track and let them drift apart. When you line them up in the middle of the track, they drift together, then drift back apart.