W3CQH wrote:
If 1 does not have access to a handbook, is there any way to determine the
value of a piece of ladder line as to whether or not it is either 300 or 450
ohm?

Lots of technical ways but as a first guess, TV ribbon (ladder line) is
300 ohms and the conductors are spaced about 8.5mm
450 ohm ribbon has wider spacing.

Alan