I've purchased some very nice cameras from folks who were sure that the instrument had a locked-up shutter. Turns out, many old cameras had a simple mechanism to prevent double-exposures of negatives; a film interlock.
This interlock was usually a very simple mechanical sequence stop. Unless you had film in the camera and had wound an exposed frame on to the next, you couldn't trip the shutter. This was fairly common on cameras with a leaf shutter out in front with the lens, where you needed to manually activate a shutter-cocking arm before each shot. If you waited too long between shooting a couple frames, you could often forget to advance the film. The interlock prevented you from ruining a precious moment.
Eventually, most camera manufacturers devised a way to wind the film and cock the shutter at the same time. But many fine old folding cameras did not have this feature, particularly German-made film cameras and some Kodak 35s. 126 Instamatic-type cartridge cameras (even 110 models) came with this feature as standard, but then, who has any film for them anyway?
To test a camera's shutter and get around the interlock without film loaded, cock the shutter arm and then open the film back. You'll see the film sprocket advance (on 35mm type cameras) and the film take-up reel. Use your finger to gently rotate the teeth on the sprocket advance in the direction of the film winding. You'll usually hear or feel a click as the interlock disengages. At that point, you can trip the shutter. If this doesn't work (and you know the self-timer has not been engaged), you might want to reconsider the functionality of the camera if you intend to use it as a shooter.
One way to research if a camera you're interested in has an interlock is to download an old instruction manual. You can find many manuals from places like Michael Butkus' www.orphancameras.com. Thank about making a small contribution to help Michael keep this very useful resource online, too.