A lot of people use a beer thief. I prefer the kind that's a long tapered tube with a small hole on top. You sanitize it, dip it in then plug the top and draw your wort out. You can usually get enough for a sample in one good pull. You can also use a turkey baster, though its harder to clean and sanitize and it's a little more difficult to pull a sample.
Get yourself on of these. It's a very worthwhile investment. Some do drop it right into the fermenter but there are two things to keep in mind. (1) You need a fermenter with an opening that allows you to retrieve your hydro. (2) You can't degass your sample--you'd have to degass the whole batch, which you are not going to do, so your accuracy will be off.