Gauging wire wheel and hub wear

Several replies to my search for a good used set of rear hubs for a 
wire-wheeled midget cautioned about risks of worn hub splines. That led me to 
realize that I have not found a reference to use to gauge the condition even of 
my remaining wheel assemblies. How does one determine that it is time to 
replace a hub or a wheel before the inevitable day that either damages the 
other?

   Butch