Tom- Yes, but there's a greater "fudge/judgement" range than if the spoke was in hand. I simply place the end point of a ruler against the spoke still in the hub, as close to the spoke's elbow (or head if straight) as you can and make a mental note of any "correction factor" if the ruler end can't line up well. Run the ruler parallel to the spoke in play and note the rim's inner point. Now add up that sort of length you got with any head end correction factor and also add the rim's "thickness". For spokes that interlace the others (the usual case) I'll add a mm or two to account for the spoke not traveling a true straight line.

If possible, I like to place the estimated length spoke against the still in the wheel one to try to confirm the length is close enough to work, before I leave on the tour... Andy

I suppose one could do a spoke length calculation if the rim, spoke pattern and hubs are known.