Phasing is electrical and mechanical.
Phase, as related to speakers, means that a positive signal moves the cone/dome/ribbon/electrostatic panel/etc. towards you and that a negative one moves it away from you.
If you take two drivers and face them both towards you, they are mechanically in phase.
If you turn one of them the opposite direction, they are mechanically 180 degrees out of phase.
Hooking up the one you turned around backwards (+ to - and - to +) will put them back in phase.
Use a mono, low-frequency signal to listen for out of phase speakers.
A stereo signal has, by definition, different sounds coming out of the two speakers and it makes it much harder to determine if something is in phase. You want a simply mono signal, preferably a low-frequency sine wave (not very low with 3/4" speakers, though!). As was suggested, listen with one ear. Move past the unit slowly maintaining a distance of about a foot. If the sound gets quieter in the middle, your speakers are out of phase.
I am not saying whether the i-opener speakers are in phase or not, but just wanted to clarify some points.