open up your script editor and chose from it's script menu: "Echo All Commands"
this shows EVERYTHING (almost) that happens in MEL. now you've done this, simply select Edit>Select All By Type>Geometry and the script editor will show you the exact MEL that this executes.
the only thing you need to realize is the the second line here is actually the MEL that is called by the scripted command SelectAllGeometry, so you could use either of those loines (but not both)
(p.s. don't forget to turn OFF "Echo All Commands" when you're done.)