How to list installed versions of the same program
You have the right command and just need to set the proper option:

which -a pymol will show all instances of the executable pymol located within your PATH.

For example, on my laptop I have git installed in two places:

$ which git
/usr/local/bin/git


provides what will be run when I use the command git.

$ which -a git
/usr/local/bin/git
/usr/bin/git


provides every instance of the executable in my PATH.