fuel gauge and sending unit check

I did a search and did not find anything but could have very easily not looked hard enough. The fuel gauge on my 53 does not work and I am not certain if it is the gauge or the sending unit. On these 24 volt trucks can you ground out the sending unit or gauge to see if it is the gauge or sending unit. I know on the old cars if you grounded it out and it went to full it was the sending unit and typically if it did not move it was the gauge or both. This kind of goes along with fuel but not so much the wiring. On my 53 REO do I need to buy a lead additive for it to help with the valves?

I guess also if it is the sending unit are the sending units for the multi-fuel the same as for the gas fuel tank? I have found them for the multi-fuel but it does not specifically say they are for the gas as well.