In warhammer 40k what do markerlights look like and where would you find them in the tau fire warrior sprues or devilfish sprues?

Well I am not a Tau player, so it's a rough guess for me. To my knowledge the marker light drones look like Gun Drones, just instead of the guns on the bottom there's a comm-like relay on the top of the drone.

Now as to where to find them on the sprues, I have no idea, Hope what I posted can help you some.