The combination of the slightly thicker modern film stocks and the ever increasing age of Super 8 cameras have made film transport the biggest obstacle to suceeding with Super 8 film these days. To be confident your camera is actually filming you need to know, more than anything else, the SOUND difference between the motor running on its own, and the sound of the motor running while the film is actually moving. This video shows various camera approaches to indicating film transport as well as including a sample of the sound difference (towards the end of the video).
You can do a few things to help with film transport. If you have stored film in the fridge, make sure it is up to room temperature before you open the foil pouch. Any condensation that forms in the cartridge makes the film sticky and causes jamming. If you have ever had issues with transport in a camera, check the cartridge for slack before popping it in. Make sure to only turn the core CLOCKWISE, gently, until you see the film moving in the window of the cartridge. If you have taken the film out of the camera and don't know how much you have used, watch this next video for a technique to get a rough idea of where you are up to.
Also, see our 'Know your Camera!' page for tips on loading rear door cameras, in particular the quite common Canon 518 SV which is prone to misloading of the film and usually requires the special intervention shown in the video on loading.