One of the easiest checks in Windows for Joint Stereo or Stereo is Mp3tag, and be sure to show the 'Mode' column, which will simply say Mono, Stereo, or Joint Stereo.

In Windows (this works from CMD not PS), for example, here's a command I use frequently to batch convert and normalize from WAV files to MP3 via ffmpeg and then use SoX to set to Joint Stereo (its default output). I wish this were not necessary, especially when working with lossy MP3 files where each action on the files downgrades them a bit, but it's not too bad. I use lame or sox to set MP3 files to joint stereo after making other changes with ffmpeg. The -joint_stereo 1 option does NOT work as a parameter in ffmpeg. MP3 players that require Joint Stereo will choke on these files. It sets the header to Joint Stereo, but doesn't actually encode in Joint Stereo. Unfortunately, this is a longstanding bug in the way ffmpeg uses libmp3lame. I'm afraid James P is mistaken, at least as recently as ffmpeg 3.4.2, but this is a common misunderstanding.

Joint stereo or stereo