1. Correct. The ones building and guarding nests are males.
2. These fish are either males guarding a nest area or females holding eggs. Most likely, they are males guarding a nest though. Net one out and see if it has eggs in its mouth. That'll tell you what you've got.
3. You can vent sex them. It's a little difficult though, unless you do it a lot. One method of ID'ing sex is to place 8 or so tilapia in a 55 gallon or bigger aquarium. Watch for males digging and guarding nests, and pull them out when you see one. Keep doing this until no one is building a nest. Ideally, you've got all females left in the tank at this point. Now, grab your best male that you removed previously and put him in the tank. Now you've got a breeder colony.
4. You can fin clip them. Simply cut a portion of the back end of the dorsal fin (the part without stiff spines) off with some scissors. It will grow back in about a year, but there will still be a slight scar. If the scar isn't enough for you to easily ID, clip the fin again. It causes no harm to the fish. You could tag them, but tagging is a bit more advanced, and can cause problems with the fish if not done correctly. It can also cause issues during spawning and injuries if the tag snags on something in the tank.
5. Breeders like lots of space. Keep your fish density low in a breeder tank.
Good luck!