How do you measure the impedance of a broadband transformer for a RF
transistor amplifier? I'm referring to the input and output broadband
tranformers just before and after the transistor finals.

You don't! The broadband transformer reflects the load impedance back to the
driving stage. In other words, if the turns-ratio is 1:1 then the input
impedance of the transformer is equal to the load impedance.

If your load is 50 Ohms and you want to match a 10-watt transmitter, with a
12v supply, then you need to calculate the impedance ratio. The transmitter
output stage impedance (in Ohms) is

(Vsupply squared) / Watts

With 12 v supply, the output impedance of a 10- watt amplifier is
(12 x 12) / 10 = 144 / 10 = 14.4 Ohms.

The transformer impedance ratio is therefo
14.4 : 50 = 1 : 3.5

The transformer turns ratio is therefore Sqrt ( impedance ratio)
= 1: 1.8 (close enough to 1:2) step-up.