You don't even need to put film in actually...
You'll need a 4LR44 battery first, which can be found in almost any film developing lab. You'll need some aluminum foil to act as spacer as well as the battery is a bit short in height before you put the battery cover back.
In theory, you can choose the lowest ASA and set your aperture to f/16 and point the camera towards something dark, the orange "SLOW" indicator should light up telling you that the resulting photo will be under exposed. You can do the reverse by choosing the fastest ASA and choose f/1.7 and point it towards a bright sky and the red "OVER" indicator should light up.
If the camera can make a proper exposure, you'll never see the two lights light up.
One common thing to look out (well, listen, really) for with the 35GSN is the very faint "clunk" sound whenever you wind the film advance lever. Sounds like a pencil eraser when you tap it on a table. That SHOULD HAPPEN on a normal GSN, if it's not, then the pad thing is worn out.
Other stuff to look out for are generic to (film) cameras... fungus on lens, whether the viewfinder/rangefinder and focus split image thingy is clear, leaking light seals (can be seen with broken foam surrounding the rear film door, etc etc etc.