I had a spinal fusion a few years ago. Do I ever need to go back and have the screws checked? Or do they stay in place forever?

Yes. You should contact your surgeon and find out when he/she would like to see you. It is important to have follow-up to assess the fusion and make sure the screws/rods are intact.
Screws stay. I hope you completed your follow-up with your surgeon. The instrumentation is generally intended to stay.
Yes. Screws are followed until a solid fusion is seen screw good at 18 momths stay good for life with a solid fusion.