HELP! glow plug relay fuse?

I need help! My dad has a 1982 300SD diesel turbo with bad glow plugs. We are going to replace the glow plugs but how do I know if the relay is still good? Is there a fuse for the relay? How do we go about changing the fuse or the entire relay? Is it OK to buy a used relay? I am so confused after looking at all the different articles on the internet - please help if you can. Thanks, Michelle