For the longest time I have had my floor-standing loudspeakers too close together. The single best test to check your speaker placement/stereo imaging is to play some live music, preferably music captured in a room with echoes. If your speakers are out of alignment you will know it and all you have to do is keep on moving them around until you get it (remember to turn off all equalization/room correction and distance settings). It will sound as if the echoes in the recording are arriving too early and clashing all over the place. You should be able to close your eyes and imagine all 4 walls of the room without any issues.
One thing to look for is when higher pitched sound comes primarily from one of the two channels then appears on the other in a delayed fashion.(this can also appear as the in-phase audio breaking imaging by ‘bleeding’ towards one of the speakers when it gets loud however this is harder to determine because the artist may just be moving around.) The Imaging should never be broken like this, if you consciously aware of the delayed reflection then your speakers are too close together and it would be a good idea to use that sample to play over and over while you make adjustments.
I have two CDs you can try out: 1994 Patrica Barber ‘Cafe Blue’ PREM-737-2, Piano Collections Final Fantasy VII (IMPORT) KO-99030, and Yo-Yo-Ma ‘Songs of Joy & Peace’ B001BN1V8U. All speakers are different and your speakers will define any toe-in. Mine are minimal toe-in and at the summits of an equilateral triangle +- 3″ and about 130″ apart(this is staggering if your not used to it because the center seating position doesn’t even feel like your in front of them anymore). Some day I will explain how toe in changes how your music is perceived.
UPDATE: Placement for Music is different than TV/Movies. MultiChannel music and TV/Movies is designed for toe-in. Music, on the other hand, works best with little or no toe-in.