Ride for a while and lift the hood enough for you to peek inside while floating... Seriously though, I looked into this heavily a while back and it's not really practical. The cheap fuel sending units are floats on a stick and are designed to work in shallow and wide tanks so they don't work very well for our tall narrow tanks. The sending units that would work, the electrical reed switches, are more expensive and usually around $60+ for the sender only then you have to buy $20+ gauge as well. So in order to make it work, you're looking at almost $100 shipped for the parts you need.
or....you can hook up your fuel switch from your stock ski and run a second line inside your kart tank. I used the return fitting as my "on" position with a slightly shorter line in the tank and the primary fitting as my "reserve". I then have to add a small barb in the tank cap for the vent barb since the old vent barb was now my return fitting.....or better yet, spend the extra money and get a custom tank built that will hold more fuel.