Pink noise 2dB louder on left is the definitive test. You could also try feeding a mono steady tone to both speakers and measuring the RMS voltage on the speaker terminals, if you have one. But it sounds like you have an "image bias" problem, as we used to call it.
I never assume a symmetrical placement in the room is going to result in symmetrical channel balance. When setting up my speakers, the first thing I check for is a rock solid mono image centered between the speakers. Unfortunately, the mono switch has disappeared from most stereo preamps and integrated amps, which complicates doing this test. Note to manufacturers: just because fewer people are listening to records, where the mono switch eliminates vertical noise on mono records, or FM radio, where the mono switch can eliminate noise from fringe stations, doesn't mean it isn't still useful.
Ideally you should get that rock solid mono image not just from pink noise but a variety of recordings. Then, if a stereo recording is biased, it must be the recording. (The balance control is another disappearing or disappeared item from stereo equipment, to my disappointment). But if you don't get a solid mono image, or unbiased stereo image, then you need to tweak your speaker placement, even if the result is assymetrical placement. My current setup in my current room is.