If you can hold it with a flat hand using just your fingers and not have it overlap very much if any, it's for a laptop. If it's about the size of your face, it's for a desktop.

Also, it's VERY rare to see IDE hard drives in laptops, and they're special ones.
Like Karaya said, it isn't going to be very reliable. With the way they were made back then, it could run for years. If it's been abused, it might die in a few days. You can't really tell.