"W3CQH" wrote in
:

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?

The most accurate way is to measure Zo as directly as you can.

Most of these lines have an actual Zo somewhat different to their nominal
Zo.

For most constructions where the conductors are mainly surrounded by air,
you can get a close idea by treating them as an air spaced line.

The calculator at http://www.vk1od.net/tl/twllc.htm allows you to supply
the conductor diameter and spacing, and it will calculate Zo (and a lot
more). If you fiddle with the value of relative permittivity to change the
velocity factor (if you know it), you will get a better approximation.

Owen