How can you tell if a recording is in mono?

If you get different sounds in left versus right, you are getting stereo. The sounds you hear in each is a result of the way the stereo recording was mixed and mastered. It is what it is.

When you hit the mono switch, the two channels are merged and the same sound comes out of both speakers resulting in a "mono" image that you gear has assembled from the original stereo.

If the recording is mono and not stereo, you will get similar results to listening to the stereo recording with the mono button/processing engaged even when it is not engaged because the recording itself is mono to start with.

This is all the way it supposed to work, so consider yourself fortunate....there is nothing wrong to be concerned about.

I never used to care for older mono recordings but I am finding many from the 50s in particular to be very enjoyable of late.