I have a ho locomotive with two sets of wheels that are narrower than they should be which causes a short at some switches. Is there a tool designed to adjust the spacing of the wheels? Preferably on the locomotive. These are truck wheels and tender wheels, not drive wheels. Also is there a decent gauge for checking them?

There is a German company that has these kinds of tools in their railway modeling tools section:
https://www.fohrmann.com/en/

I would imagine that US model railway shops/companies will have similar tools?

Fohrmann has a rail gauge tool that measures wheel spacing, 13.8mm for Maerklin and 14.3 mm for DC. But you could also use a calliper to get precise measurements. Take note that 13.8 is on the small side, 14mm should be better. At least compare the wheel spacing on some of the other axles.

On a side note I believe that I've been able to increase the wheel spacing on a locomotive with hand tools without removing the wheel set... with the utmost care not to damage anything of course.

I have some of the Fohrmann tools and use them a lot on non Maerklin cars and waggons.

Posted at the same time as the answer above :-)