How to have an object check to see if a set of position are free of other gameobjects, then randomly choose one.

I want a script feature that has a customer come in make an order then go to an booth to sit at that isn't occupied by another customer. I have most of it down other than the "see and choosing a spot" mechanic.

I have a script keeping track of the spots that are opened or not in a list of strings, adding and removing spots when customers leave and stuff, but I don't know how to get a randomized output from the list string to tell the new customer where to go.