The coin comparator is a device that is located directly inside and under where you insert the coin in the machine. Every tokenized $1, $5, etc. slot machine has one that holds the houses token inside the comparator. When you drop your token through the slot it compares your to theirs and if it matches you play, if not it drops. This technology is similar to bill vaidators in vending machines and so on.
As far a sorting the foreign coins that is the job of the drop teams typically. As tokens are gathered from the slots, counted then re wraped or racked watchful eyes spot the foreign set them aside then are held separately in the cage. Then there are actual companies that go around to casinos buy their foreign then sell them back to the original casino. These companies are also responsible for destroying what we know as obsolete i.e if they have no casino to sell them back to there detroyed. Hope that answers your questions, Eric