Yeah, I know, just listen carefully, but with a lot of early live tracks, and even some early studio tracks, it's not as audibly obvious as you might expect. I found that loading the tracks in audio editing programs to show the left and right channel wave forms is one way. In mono, both appear the same, and of course they differ in stereo. Can anybody suggest any better "foolproof" methods to be 100% sure?

Listening on headphones works for me.

Audio editors are the 'bulletproof' method for me. Zoom in on a section until you can see very separate waves

No disrespects to anyone here, but if you can't actually hear whether it's in stereo or not then why do you even need to know this?

It's very simple. I have some old stuff from the 60s in flac. All very good, but playable only on computer. I'm converting to mp3 (I know, some quality loss) so I can burn to disc and play on DVD player on a sound system (loafing on the lounge instead of on a stiff computer chair). For several of the files, I have no information on whether they're stereo or mono, but I want to do my converting to mp3 in line with what the flacs actually are (whichever it is - very hard to tell with some old stuff). Just another piece of perfectionist work - I'm sure many will go along with that.