How would you know if its your freeze plug?

Freeze plugs are on the side of the engine block. They are round and aprox 2" in diameter. They can rust and leak. If you still can't find the leak, purchase a dye for the cooling system. Follow the directions that it comes with and where the dye leaks from, you will see it with a black light. Also, you could simply pressurize the cooling system and check for leaks. The tool needed is nothing more than a hand held pump that most parts stores will lend you.