I use both methods, the plastic hydrometer tube or just dropping the (sanitized) hydrometer right into the fermenter depending on which fermenter bucket I'm using. One of my fermenter lids has a hole in the lid big enough to pull out the airlock stopper and look inside. I use a sanitized turkey baster to extract some of the brew and put that into my hydrometer tube to take a reading. If I'm using my other bucket, the hole in the lid is too small to extract any brew out of it so I just take the whole lid off and drop the hydrometer directly in. Sometimes that's it's tough to take a reading that way if there's still a good krausen layer on top, but if there is then fermentation probably isn't finished anyway.