On an Ubuntu system there was some software that I couldn't remember if I installed it using the apt package manager, or if I used Snap. I thought it was snap.

All I would have to do was run the command that lists all installed Snap software. I found that you can get a list of software installed with Snap by running the following command:

snap list

And here's the output from that:

root@somewhere:/var/lib# snap list
Name    Version    Rev    Tracking       Publisher   Notes
core    16-2.44.1  8935   latest/stable  canonical✓  core
core18  20200311   1705   latest/stable  canonical✓  base
lxd     4.0.0      14709  latest/stable  canonical✓  -

So running that "snap list" command gives a nice output of all the snap packages installed, complete with their version number, revision, tracking, and publisher.