What signifies a guest is done eating?

Normally when a client is ready for the bill they will signal you to get your attention and ask for the bill.

Otherwise, you wait until their plate is empty and they have put their cutlery down. If someone else is still eating at the table let them finish as you don't want to appear to be pouncing on them every few minutes.

If the plate is not empty because the person is full the knife and fork are normally together side by side on the plate rather than at each side of the plate. At each side of the plate signifies they have put the knife and fork down, or whatever, to do something else with their hands. In the closed position means they are done.

Basically, always keep an eye on your clients. When in doubt, give them extra time to finish. No one likes to be rushed out of a restaurant.