It’s easy to test. Mount the flash on the camera hotshoe, make sure it is set to TTL mode, and fire a shot with the lens cap on. The camera should briefly display the amount of underexposure (-3ev in this case). If you then press the button combo (select and mode at the same time), it will recall this value. If you’ve switched off the flash, or if your last shot was exposed properly it won’t show anything.