How do you measure the length of a loose spoke?
Generally speaking, the unstretched length of a spoke should be measured from the inside of the J bend right down to the far end of the spoke.

Your best tool for doing this is either a metal ruler where the 0 measurement is right at the end of the steel, or a normal ruler with a notch filed in the end, down to the 0 line.

The best method is to hook the J of the spoke over the end of the ruler, and then read off where the other end falls.  Accuracy to the millimetre is fine, no need to go any more precise.

Here's the Park SBC-1 which does exactly the same thing.

Since spokes are sold in metric lengths, you'll always use the metric side.  See how the J is hooked into the hole at 0?  This one is about 268 mm long.

Another method is to simply bunch the spokes in a fist and push the threadded end against a flat surface.  The longer spokes will poke up more.



Things of which to be careful:


the amount of thread rolled onto the spoke could be different between types
the thickness / gauge of the spoke could be different, giving it different flex characteristics which could move load to this spoke, or away from this spoke
Gauge of nipples should match gauge of spoke too, else they could back themselves out over time, or make tensioning harder.
Finish - spokes may be galvanized, chromed, blacked, or coloured.  Some riders care about this kind of thing.
Set - a used spoke will subtly vary the J bend at the hub.  This can stress the used spoke and make it snap easier.  It can also sit differently in the hole, and may adjust over time causing the wheel to untrue erratically.
Nipples - have a defined tolerance for damage.   If anyone has monstered the nipple with the wrong size of spanner then it may be hard or impossible to tension the wheel right.  If possible reuse the nipple from the broken spoke, assuming its serviceable.


Personally, I have no problem reusing an undamaged salvaged spoke.  I've also rebuild a whole wheel onto a new hub reusing all the spokes in that wheel.   However none of these were high performance wheels being pushed hard.

Use your best judgement of course.