With regard to effects pedals it’s not critically important
Tonmann did a post on the old forum explaining how to work out if your input was in or out of phase with your output
This is a good explanation taken from this site
Audio waveforms are cyclical; that is, they proceed through regular cycles or repetitions. Phase is defined as how far along its cycle a given waveform is.
The measurement of phase is given in degrees, with 360 degrees being one complete cycle.
One concern with phase becomes apparent when mixing together two waveforms. If these waveform are “out of phase”, or delayed with respect to one another, there will be some cancellation in the resulting audio.
This often produces what is described as a “hollow” sound.
How much cancellation, and which frequencies it occurs at depends on the waveforms involved, and how far out of phase they are (two identical waveforms, 180 degrees out of phase, will cancel completely).