Without any RF test equipment, another way to tell is by physical
measurement of the dielectric and inner conductor diameters. But you'll
need some calipers to do that. If you have piece of marked cable of the
same outside diameter and either impedance for comparison, you can
easily tell the difference -- for the same outside diameter, 75 ohm
cable has a noticeably smaller inner conductor than 50 ohm cable.

Roy Lewallen, W7EL

Jack Twilley wrote:
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This is a little bit off the original topic, but I have a question.

Given two unmarked pieces of coax, how do you determine if they're "50
ohm" or "75 ohm" or whatever? Shorting them doesn't work.