Either listen in mono, or pan both to center....depending on how out of phase they are, you'll hear anything from a swishing noise as the phase cycles overlap periodically, to one or the other dropping out almost completely (180 degrees out). Bear in mind that it's normal for various sound sources to not be perfectly in phase with one another, it actually contributes to the perception of a stereo field.

PS: Phase alignment, or "coherence", is a function of time. If you take two signals of the same frequency and they start at slightly different times, they're out of phase. A phaser stomp box works by splitting the incoming signal and sending half of it through a (very slight) time delay circuit, then remixing it with the original signal before output.

If your talking about two seperately tracked guitars, they are not really related and can't be "in" or "out" of phase with each other. If its the same guitar tracked thru two differen't inputs like a DI and a mic they can be in or out of phase to each other and the sum to mono trick will show this. Out of phase at this point means that the waveform on one track will cancel frequeincies in the other track when summed. If you have a daw a simple experiment will demonstrate phase. Take a single audio file in a track and copy it to another track making sure the start points are exactly the same. Using the invert pluggin in your daw invert one of the regions. If you zoom in now you will notice that the waveforms in the two regions are mirror images of each other, where one wave ascends, the other file's wave is descending. If you pan these hard left and right and hit play you will hear a weird (phasey)sound. Now pan them center. While the track meters in your daw will show level there should be absolute silence from your monitors as the two tracks are 180 out of phase and will cancel completely in the daw. Now slide one region 1 sample. You should start to hear your audio.

so If I record two DI guitar tracks I won't need to worry about phase problems? but if I record one DI and one Mic'd I may have a problem?

thanks

Sounds about right. Time delay creates phase cancellation, the time it takes the sound to get from the speaker to the mic is the main source of delay.

If you are talking about 2 Di's from the same source guitar you should still check for phase with two DI's. You just don't know if a cable is miswired or a DI or something else in the chain is miswired. Pan them to mono and if the signal gets louder they are in phase and if it gets quieter it is not in phase.

If you are talking about two differen't guitars then phase usually will not be an issue no matter how you input them, DI or mic. However, if you were miking two acoustics in the same room, and the two mics are known to be in phase with each other, you would still get time induced phase introduced from leakage between the two mikes, and the only fix for this is isolation of the guitars being recorded.

The time delay induced phase from a mic and a DI on a single recorded source can be easily remedied in your daw by sliding the miced audio region back in time to line up with Di'd version at the sample level. You can also use this as an EQ by moving it around at the sample level and listening to the result. Really changes the sound with a bass or guitar.

Any polarity "phase" issues can also be easily remedied after the fact by inverting one of the tracks for playback, i.e. most eq's have an invert switch or an invert plugin. If you find you have a miswired mic or di you can make a 6 " mic cord and reverse pin 2 and 3 on one end. This is also a handy phase bender when micing the bottom of a snare.

even when you get the Polarity right the DI signal and the Mic signal can never be completely IN phase for the whole frequency range as the speaker and the mic and the DI all have a different phase response

that's not to say you can make things sound better when some thought is applied to polarity and phase

I'm recording two seperate guitar tracks using the same guitar and same DI. Could this cause phase problems?

thanks,

No more than two guitars with the same set-up and the same type of DI and the same guitarist
(yeah I know, but you get my point)

It is a very interesting discussion point and has been discussed in white papers ... things like ... distortion of massed strings and chours.

I don't think you need to worry about this as it is now drifting into not what is correct but what sounds better.
What sounds better is your choice.

hope that helps a little