There are a couple main ways to identify these fish apart.
Here is a Dogfish nossle.
And here is a Bull Huss nossle:
The first identity method is to look at the nasal flaps of each fish. The Bull Huss nasal flaps are separate and not continuous with the upper lip like the Dogfish.
The other way is by colouration, the Bull Huss generally has larger spots and is usually a darker colour with dark blotches underneath unlike the dogfish which is completely white underneath. This picture shows a Bull Huss on top and a Dogfish below it.