Halibut have teeth...a different tail shape...and the line down the middle of the fish (both on the brown side and the white side) has a distinct hook (or curve) up towards the top; the sand dab, sole, flounder or turbot all have a straight line from head to tail.
The warden checked all three (teeth, tail and line) last time I brought a load of Sand Dabs in...and he checked every one of the fish and I had about 30 of em'!
This is a Halibut...notice the curved line near the upper fin...
This is a shot of a Halibut tail...notice the curves as opposed to the flat edge of the other bottom fish...
This is some Sand Dabs, notice the straight line and the flat edge of the tail.
Good Luck
Jim / Saba Slayer