How to test balance between right and left channel

A SPL meter plus a source that provides separate left and right channel signals. I believe there are cell phone apps that provide common SPL meter functions.

If you have a mono source you can measure the output of each channel, BUT measuring the output will tell you nothing about the system’s music balance as the measured signal cannot take into account the room’s acoustic properties.

Brf is correct. In my experience even though the equipment is balanced equally from left to right the room is where the problem lies. This can also be frequency dependant.

Thanks for your replies. Bfr/Rrog, you may be absolutely correct. My dedicated audio room has a sloping roof - 8ft high on one side and up to 20 ft on the other. But before I tackle this problem, I need to be sure that the slight imbalance I am getting at time does not come from my equipment. I know my cd is ok as i can read my the balance levels on it. But i need some sort of equipment to individually test each component of the chain to identify the source of my problem.