Twisted pair impedance
Do some time domain reflectometry, a useful video to this process is probably one from w2aew. He shows it with a coax, but it should work with anything that has a somewhat consistent impedance.

In short:


Inject a pulse with a short risetime into the cable
terminate the cable with a potentiometer that has the range of suspected impedance (like 200Ω)
Watch the pulse being reflected on the scope. Adjust the potentiometer until you have minimized the reflection
Your potentiometer is now approximately the impedance of the cable