Pull the sender and measure the resistance, it should range from about 20 ohms up to 220 ohms at each end of its swing.

 

If memory serves, the correct one has a longish straight arm to the float, whereas the one I was first given had a 90º L bend in it, and was shorter.

As regards figuring out fuel level, I have no proper idea, BUT I would pull out that spare tire, and the trunk back board. There is then the access to the sender... Get lots of rags, soaked in water.... use the brass drift (see the post further up this thread)..... and GENTLY start to remove the securing ring. The gasket might be tight, and the sender may not be loose immediately and could need some encouragement. IF fuel starts to seep out get rid of the cigar, and re-secure the ring.
At least you will know that you have c. 2/3rds to 3/4s fuel.
Others may have a more sophisticated method. I will defer to them !!!

 

I've cracked mine before with it over the sender level and its not THAT much fuel that leaks out. You could always stick something down the filler neck to check.