With a built wheel that's missing a spoke, it's sometimes possible to measure along an existing spoke on that wheel. Chose a spoke that is on the same side and orientation as the missing spoke. Measure from inside the bend at the hub to where the nipple enters the rim and add about 3mm. As mentioned by others, you can measure to the end of the nipple and add its length to the measurement, but you need to know the length of the nipple for that. You can sometimes do this for straight-pull spokes too if you can see where the head is, and measure from inside the head. Hidden heads, internal nipples, proprietary spokes and nipples, etc. all complicate matters.