I just bought an old Mamiya 500tl and from what I can tell it works okay. I've taken few shots but when I open the back I cannot see anything on the film. Should I be able to see see the negative? Is there an issue with the camera? It'll be pretty funny if I took the film to get the pictures and there were none. Can I test it somehow?
You won't be able to tell by looking at the film. When the film is exposed, the changes in the crystals in the film's emulsion are invisible, and the exposed film is said to contain a latent image. This image is made visible by developing the film into negatives from which you can make prints (or for slide film, into slides). After this, a fixer is used to stop the film from being further sensitive to light, making the image permanent (or as close to permanent as reality allows)
Unfortunately, this fixer stage hasn't happened to your film, so when you opened the camera to check, you almost certainly exposed the film directly to light, which would cause it to become entirely exposed (wiping out any latent images). I hope none of your test shots were important.
You can do a very basic test by leaving the back of the camera open with no film installed and watching what happens when you press the shutter. I'm not sure how this particular camera works, but you'll probably have to cock the shutter and then will be able to release even with no film. Go through the shutter speeds from 1 second on down, and watch what happens. You should actually be able to see the shutter open. If this is all working okay, and if the film is properly advancing, there's no reason to think that it isn't capturing images.
Most photo development places will only charge you a token amount for blank frames, so it might just be easiest to shoot a test roll (without peeking!).
I know it's a little hard to adjust to this after being used to the instant feedback of digital, but the delayed gratification can also be part of the enjoyment.
When you open the back of the camera in an environment with any light present you expose the film to additional light and the film becomes totally exposed. At that point you have a solid black negative that will produce a solid white print. Only after film has been developed and chemically "fixed" in the dark can it be observed without the light used to observe the film not creating additional exposure!