Rider 1-42; Picked up a nice 1929 radiola 66 from orginal family, radio plays good. After cleaning & testing radio tubes,started placing tubes back in. I noticed in the 280 socket, a third hole was enlarged allowing 2 positions to plug tube in.My question is; how can I test with a ohm meter so 280 tube filment pins are correctly plugged-in .?? 1st time for this problem!

The two pins with the Least resistance would be the 5 volt filament winding. This is where the two fat pins go.