See if you can get some of it on your fingers. Brake fluid is oily and antifreeze smells sweet and is also oily…HOWEVER anti freeze has no business back there so that may just be evidence still left behind from a previous persons car.
Also if you are leaking brake fluid you will quickly notice an issue with your brakes. Your brake pedal will begin to go further down twd the floor and you will have reduce braking power. You will notice this rather quickly if you have a leak so…if it is brake fluid you prob already know it.