To measure a spoke in place you take a ruler and place it against a similar but good spoke. You measure from the rim's inside surface to the hub's outer surface. THEN you determine the amount of the spoke that is hidden in the rim and overlap/is inside the hub shell. At the rim end this can be a few MM's to many depending on the rim/spoke/nipple design. At the hub end this can be the same, a few MM's to the head's inside to many MM's to the end of a threaded into hub design.

Because the amounts that you can't see are hard to determine the best method is to remove a good spoke and do a direct measurement. Some bike brands have this spec listed in their technical sheet (if the wheel was an original spec), some wheel builders keep the spoke lengths on record. Andy.