The first step would be to check the Infinity website. They have a small section decribing which speakers are matched.
A good movement DVD should help show how well your timbre matches. look for one that has a specific movement (the A10 napalm attack in Courage Under Fire is a good test) rather than one like SW Ep 2, which has so much movement that it's hard to local a single movement. the Phalanx gun attack on the F18 in Under Siege is another good one.
listen for a smooth transition of movement. if the sound doesn;t quite follow the screen, then your timbre is out somewhat.
a couple of tuning tricks to help (not solve but help) matching timbre is to move the center speaker (or alternately the left and right mains) forward and backward a couple of inches to match the timing a little better. you can also try setting the receiver to small and large for the center to see which sounds better. unfortunately, this particular trick depends alot on your source. on my system, the center works best on large for movies, but small for Fox's 5.1 football broadcasts.
timbre matching is less important on your rear speakers. in movies, there is alot more space to transition and usually the sounds are a lot less defined. if you plan to go to (or are already at) SACD or DVD-A music, it makes sense to buy a higher-end rear (either dipole or bipole, but you;re going to need someone smarter than me on that particular subject) and not worry about how the timbre matches as much.