I had my forks serviced recently, as the seals had perished and oil was leaking out. When I picked them up, the guy said he had also fitted new bushes as the existing ones were damaged. I understand what the seals do, but what's a bush for?

Do they have to be replaced much, and how do I know they are really replaced. The guy showed me some metal rings but how I know they from my bike (he had a whole bucket full of all sorts of scrap metal)?

Although there are many components in a set of forks, there are only really two bits when assembled, the outer tube that is clamped in the yokes, and the stanchion that's fixed to the wheel.

While the seals keep the fluids inside the fork apart while allowing them to slide over each other. They're normally made of steel covered with copper and Teflon to reduce friction.

As well as allowing the fork to extend and compress, the bushes have to work under extreme pressure. So it's even more important they have low friction. When you're braking hard, the fork tries to bend, and all this pressure is passed from the tube to the stanchion through the bushes, as they're the only point of contact. So it's important to change them if damaged.