another simple test is to reverse the + and - speaker leads and see if the center image gets more diffuse or more focused. obviously if it gets more diffuse the original orientation was in-phase. be careful as not all program material has center channel info so the result could be misleading. i would make sure the recording you use has clear center channel content (voice is best).
personally; i have recently added multi-channel to my dedicated 2-channel room for SACD multi-channel. i feel strongly that a center channel for music only detracts from optimal performance for both multi-channel and 2-cnannel. i use an analog 6 channel preamp for my multi-channel......and have had it wired to create a 'phantom' center. the center channel signal is split and added equally to both front left and right channels. this gives me at least as good a center image as having a center speaker and eliminates the negatives of the center channel.
personally; i have recently added multi-channel to my dedicated 2-channel room for SACD multi-channel. i feel strongly that a center channel for music only detracts from optimal performance for both multi-channel and 2-cnannel. i use an analog 6 channel preamp for my multi-channel......and have had it wired to create a 'phantom' center. the center channel signal is split and added equally to both front left and right channels. this gives me at least as good a center image as having a center speaker and eliminates the negatives of the center channel.