BNC connectors: 50 or 75 Ohm?

[Merlin3189]

Quick question. I'm just sorting out some spare parts and have a lot of BNC connectors. I know most are 50 Ohm because that's mainly what I've used in the past, but I think some are 75 Ohm.
Anyone got tips on how to tell them apart?

(They are generally not attached to a cable, so I can't just look at the cable markings. Some plugs have manufacturer's markings, but most don't.)

I thought I could just measure some diameters, but I think I'd need to know the dielectric constants as well and I can't see any way of determining that.

 

[davenn]

Usually, the centre pin for the 75 Ω connectors is thinner

People have damaged many a 75 Ω socket by connecting a fatter 50 Ω connector into it


Dave

 

[Arouse1973]

If you look at the ends of the connectors is the white dielectric different? Are there any that don't come to the top of the fingers?
Adam