How I do it, and this just works for me, is a two step process. First, I take a compass to the pickup. It's either 'North' or 'South'. Then I hook up the pickup to my old analog VOM. Set it for R x 1K and you might get a reading of '6' for a single coil or '8' for a typical PAF. Now take a small flat blade screwdriver, and gently tap any pole piece on that pickup. The reading will either deflect 'upwards' or 'downwards'.
Pickups that are both 'North' and deflect 'upwards' (as one example) will be in phase. You can also look at how the leads come off of the bobbin on a Strat-type pickup. Facing you, it should be the 'hot' or white colored lead on the left and the 'ground' or black colored lead on the right. If the black lead is on the left, that's likely a reverse wind. And if it's a reverse polarity, you should get a hum cancelling effect when used with a 'regular' pickup.
This isn't 100% written in stone, but the typical pickups I buy follow this. Keep in mind some manufacturers seem to get a sick thrill out of being @ssholes and not following convention. I know mixing Seymour Duncan and DiMarzio Strat pickups never works. Sometimes you gotta wire one pickup with 'white' as 'ground' just to make them get along.
If you can check all that, you'll save time taking strings off and removing the pick guard just to muck with the wiring when you have phasing issues after soldering all morning.
