You'll know if you're out of phase. It just sounds thinner, and odd.. Try making things out of phase, and listening to them.. Take a track of something, then duplicate it and nudge it back or forth about 16ms.. This will give you an idea of something thats directly out of phase.. using 2 different mic's in different positions and making sure they are in phase is a bit more difficult but not too much.. just make sure not to put one mic right on a sound source, then put another 2 feet back from the same source.. Try putting the "far" mic back 6 to 9 feet.. put on headphones while mic'ing something and put one mic about 90 to the left and the other 90 to the right.. you'll hear when it just sounds good..
if you're using 2 mics at the same distance, try using an XY pattern.. here's a picture of XY pattern..
http://www.personal.psu.edu/users/d/..._micplace.html
eric