My experience with all my indoor grows is that the closer you get to the tip of a shoot the more hollow it will be. My plants when harvested, are almost exclusively hollow with the exception being the bottom of the main stem. I have always attributed it to the fact that the plants are basically "babied" their whole life, thus no necessity to solidify the limbs.

I have noticed on the few outdoor grows that I have participated in that more of the branches were solid all the way through. Again, it fell in line with what I had observed in the past so I had no reason to question it. It's been one way I have been able to tell the 2 apart with some degree of accuracy.