I don't know why I was watching this, but in this video he measures the resistance of the pickups at the end of the guitar cable. So assuming this is accurate, I guess you don't have to remove the pickups to do it. I tried to have this start at that part.
It works, but the pot values will affect the measurements. I've forgotten more electronics theory than I remember, but the pots are in parallel with the pickups, so a 250K pot with a 10K pickup will yield a 9.6K measurement. 500K with 10K will give you 9.8K.
Probably close enough for Rock n Roll
From my Communications Tech background, I tend to like disconnecting components before measuring to avoid unknown factors and assumptions, but in a working guitar with simple wiring, I guess you don't really need to unsolder to get an idea of the DCR of the pickup.
