Can't get the trailer lights to work. Blown fuse? Corroded wiring?

I have the right coupler (4 wire connecter) and I used the old trailers lights and then bought new trailer lights - neither worked of course. Then I am thinking that it is a blown fuse. I found the fuses but how do I tell if the fuse is blown? I can tell easily enough with the 20A but what does a blown 30A or 40A fuse look like?

Help, I need to get the trailer and boat out of my driveway or the bastards in my town are going to give me a summons, or some crap like that.

Thanks guys!

Cheers,

Perturbed Pete