If you like to back up what you’re hearing with visual feedback, you can always turn to a dedicated stereo meter plugin.
A goniometer displays a signal’s width (ie, stereo balance and phase information) on a circular or semicircular interface, with the vertical axis representing mono and the horizontal axis displaying stereo content. A 100% mono signal will be displayed as a single vertical line, and stereo information that’s reasonably in phase will ‘fluff’ outwards and populate the inner marked 45-degree ‘safe zone’. The more out of phase the stereo content, the more it’ll breach outwards into the side areas. If your signal strays too far into the ‘danger’ regions for too long, your material is too out of phase, and may be lost when summed to mono.
A phase correlation meter, meanwhile, is a simple horizontal or vertical bar meter. Its values range from +1 (representing a ‘fully correlated’ or centred signal) to -1 (completely out of phase). Again, if your signal sits inside the ‘danger’ minus area for too long, you may need to revisit the sound.
iZotope’s Insight features a decent scope display and phase correlation meter