Problem

[DJASIANNOOB]

Lets say I have two songs playing, and I can hear the basslines coming apart, and getting out of sync. I see that most people will just give there turntable a tap and it will sync again, but my question is how do I know which song is to slow or fast?

[aaron altar]

Practice, which requires a lot of trial and error. When you're first learning make small adjustments and if it gets better do it again, if it gets worse go the other way. This is the hardest part of beatmatching that takes years to master. I find it to be easier with one ear in the headphones but after a while you won't need to do that all the time, you can tell by just listening to what's coming out of the speakers. It's also easier if you pick something out, usually a clap or high hat, and listen to see if that is in tempo, if it's not, is it too fast or too slow. No one is perfect all the time, just keep practicing.

[light-o-matic]

It's not just the bassline that gets out of sync, it's the whole track. The bassline/kick is just what stands out the most.
Listen carefully and with practice you will start to hear which is behind and which is ahead. But it's tricky sometimes, I've seen even very good djs mess this up once in a while. If you're not totally sure then just make a really small correction and see if it gets better.. if it gets worse then you have the wrong one ;)

If you have this problem often of the track drifting very far off, very fast, then you are not being anal enough when you beatmatched them in the first place. If you have extra time between tracks then let the two track play together in your headphones and keep fine tuning it until its nearly perfect.

[TheRabbitMonk]

Push it one way if that's not right push it the other. That fails practice some more

[DJArmani]

This technique just comes by ear. The way I learned is take Track A and put it louder than Track B to see if the bassline is out of place. If it sounds faster or slower then mess with it to speed it up (touch the turntable, speed up the pitch, etc.). Also try mixing with steadier bpms first (128 --> 127 --> 128 --> 129 etc) so u can work on adjusting pitch and blending more then you can move on to more difficult challenges.

[KLH]

This technique just comes by ear.
... and experience. Practicing really helps, but only knowing your tracks will save your azz!

-KLH