and how would one go about figuring out how many amps they are pulling?
Figure out the draw of every electric accessory and add them all up?

But seriously......
I don't know too much about wiring but I do know that fuses are used to protect the wires from overheating. Basically if you have more current going through the circuit than the wire can support then an inline fuse rated at just below the wire's capacity will blow before the wire ever has a chance to get hot.

So, theoretically you just need a fuse that is rated at a slightly lower load than the wire is.

You don't want to base your fuse on how much current you're actually drawing through the wire, but you should be choosing a wire that will support the amount of current you are drawing through it.

I hope that makes some sense.

Either way, 1/0 gauge wire will handle a hell of a lot of current (anyone have any specs on it?).
I'm not sure that you'd need that big of a gauge of wire unless you have some crazy electronics going on, but it can't hurt.
If you are running a 150amp fuse and it isn't blowing, and the wire is rated at least 150amps then you should be fine.

-Riggs.