I have used two diffferent ways. First I went to NAPA and got a small brass 3 way valve (~$10) and put it in the fuel line before the carb. I put a clear plasic tube on the third port of the valve for sampling and ran it into one of the overflow bottles for safekeeping. All you have to do is turn the valve handle to get a sample. It also works as a shutoff for fuel to the carb when you're working on it. It looks kind of hokey but it works.

Also there is a supplier (don't have the flyer with me right now) that is selling quick disconnects that you install in the fuel line. When you want a sample, you pop off the disconnect and pop in a hose with a male fitting on it. The fittings are supposed to be SCCA approved because they internally close the port before breaking the connection (no leaking). They are a bit expensive (roughly $60 for the 3 fittings, 1 female, 2 male) but they make a very clean installation.