Hi and welcome to the AOC.
If you have parking sensors you should see 4 small circles on the rear bumper, and also when you engage reverse gear you should hear a beep in the cabin. If the circles are there but you don't hear a beep, there maybe a loose connection somewhere. Check the wirering the reverse light (where the reversing sensor system gets it power), and check the speaker box which is usally located near side boot, up near the car's speaker.