How do you look inside? Visually as far in as I can see? Or take a sample?
One way is to tease back a bud mid way on the stalk, and take a peak behind it, (with a scope, or cellphone on magnify) around where the stem connects to the bud, if the buds are slender enough to allow it.
If you've got one of the donkey looking cola's, you might need to kinda bend the cola a little, and look into the inside , between buds. That's how I do it anyhow. I'm getting better at simply feeling the entire stalk and judge things from the outside, but it certainly takes practice, especially with differing cultivars. I also try and judge the plant overall, from top to bottom. Inspect overall density.
Sometimes, a cultivar will finish on the top, and might harvest a little better in layers. Taking the larger ripened upper chunks off first, then leaving the rest for a few more days to finish in the lower sections. That's usually caused by a lack of full light penetration. That's always an option also. There's really no set rules actually, I don't think. Depending on environmental conditions, the same cultivar could finish longer, or finish on time.
I suppose it could even finish earlier for some reason, but that wouldn't usually be a good thing, in my mind. It could indicate a problem. Plants generally follow along a predictable time line, and if something is happening too quickly, it usually means it's shutting down for some reason or another. If that makes any sense.
Keeping the plant healthy and happy until it's carried over that goal line should be the main objective, IMO. Toward the very end, it's going to require much less water, and it's nutrient needs will drop off also.
Just sunlight, good air circulation, low humidity and cooler temps, if possible.
