Set up the camera on a very rigid support. Perhaps a piece of wood screwed to the underside of the camera and then clamped on to a solid table. Find a magazine or newspaper and tape it to the wall. Focus carefully, then shoot about 10 seconds of film. If it’s a super-8 camera that’s it. You’ll see the result when processed. Actually it’s best to wait a few days after it comes back in case the film is still ‘green’ with some moisture in it. On projection you can easily determine whether the camera is acceptably steady. Adjust the projector’s framing so you can see the frameline between the images, and watch the newsprint to see if it moves against the frameline. If the frameline itself moves it’s probably an unsteady projector ! By ‘acceptably steady’ I mean just that. The design of the film cartridge and the way the film is transported through the camera gate, normally does not give an absolutely rock steady image. Individual super-8 cameras do vary however, and you could be pleasantly surprised.
With regular 8mm and 16mm you have the choice of a more critical test. Because the film is normally wound on a roll it’s fairly easy to wind it back and do a second pass. So any unsteadiness between the two images will show up clearly. If your film has backwind it’s very quick and simple (keeping the lens covered !). Without backwind the camera has to be opened in the dark and the film rethreaded. Using this superimposition method it’s best to underexpose each pass by one stop, so the combination is correctly exposed. It is not necessary to align the images exactly. The idea is that the small print in the double image will dance around slightly if the registration is not perfect. A vertical jitter could be due to the claw in the camera not working properly, or more likely the pressure pad needs adjusting. Horizontal unsteadiness is usually due to insufficient pressure of the side springs in the gate, if fitted. A 16mm camera in good condition should give a rock steady image. 8mm cameras can be very steady too, but are not usually as good because the projected image is magnified much more, and also the fact that most cameras are not sprocket-driven. The film is pulled through the gate only by the action of the claw.
Once you are clear about how steady your camera is, you will know its limitations if they exist. A camera with less than perfect registration should not be used for intricate superimposed effects, and it may not be too good for static shots on a tripod. On the other hand it will likely look fine for handheld shooting, as the natural movement in the image camouflages any unsteadiness in the camera transport.