Here's an alternate suggestion for checking left/right hookup: If you own a classical recording containing strings, the violins are invariably on the left. If you also use a turntable, this test will also tell you if the cartridge's right and left channels are hooked up properly (provided everything else in the chain is OK).
Also check that everything is in phase. If you have a recording of a solo singer i.e. without any accompanying instruments, the voice should appear to be clearly coming from dead center (provided your listening spot is exactly between the speakers). If not, and the voice's placement is vague, 'floating around' or coming from both speakers the speakers are out of phase.
Proper phasing of one's components is, in my opinion, more important than getting left/right correct. (The exception to this is in recordings of massed strings where a left/right channel reversal will result in the violins placed unnaturally on the right, as if the backs of the instruments were facing the listener.)
Improper phasing, which is probably more common than it should be, will result in--among other thing--a lack of concise instrument/voice placement, and loss of bass.
So when checking left/right, also check phasing--especially if you have phase inverting components (check your owner manuals) in your system. I once even encountered phase inversion with a highend cartridge, leading me to suspect that a cartridge with right/left channels internally reversed may also exist.
