How to measure the actual length of a long twisted pair?

From a purely geometric perspective you could calculate the length using the helical length equation.

Where H is the length of the twisted wire and

Where R equals the radius of the turns in the wire. Basically from the center of the twisted pair assembly to the center of one of the wires.

So if the wire makes a complete rotation around the center in 10mm, and distance between the center of the twisted pair and the center of a wire is 1mm, then were you to untwist the wire and straighten it the length would be

Cut off a 1 meter piece, remove and straighten one of its conductors, and measure the actual length per meter.

Update:

Straighten out one of the conductors and measure it. Say it's 1.05m long, or 5% (made-up number - I've no idea whether it's realistic) longer than the cable it came out of. Apply that extra 5% to the length of your cable to get the length of the conductors inside.

One easy and obvious way is to connect the cable at the end and measure the loop resistance with an ohmmeter. Of course you need to know the resistance of the cable.

The better and very precise way is to use a time-domain reflectometer (TDR). This device sends an impulse into the cable which is reflected at the (open) end. The time of the reflected signal is measured and because of constant wave propagation, the length of the cable is calculated.