Look at the other spokes. See how many spokes it crosses before it reaches the rim from the hub? If it crosses 3 other spokes before it connects into the rim, that a Cross 3* wheel. If 2 spokes - Cross 2. Etc. Notice how the last spoke it crosses it goes UNDER? If yes - Good! That means it's laced. Now do this same thing with the new spoke.

* - we abbreviate this as X3