I have a Weber 500 carb and a Edelbrock manifold. On it is some sort of switch. I think it might be a temp switch that can be used to turn the fan on and off. Can anyone confirm this???

More likely a variable temperature transducer for a water temp gauge, assuming it's sat in the water jacket

I've got the same set-up and have a sensor in same position.
I just assumed its the sensor for temp. gauge (mine isn't wired up yet.)
There is a mod whereby the stat housing is removed and a small hole drilled either side into manifold jacket where sensor sits which aids a more even temp. reading.
Its described here. http://how-to-build-a-pilgrim-sumo.wikidot.com/engine-plumbing
In fact that site is good for info on setting up your eddy carb as well.

HTH,
Andy.

I have seen that Pilgrim Sumo build guide before. He drills a little hole somewhere and that area and says it helps with hotspots within the manifold. There is some good info on there actually.

I was hoping it was a switch rather than a gauge sender

Yep it's the temp sender for the water temp gauge.

Not so long ago wired mine up...