Thermosiphon action is going to be pretty much dead silent - it isn't all that fast so you aren't going to get any plumbing noises... As mentioned, about the only way you will know it's there is to monitor your temperature changes...
One can do a home brew draft gage with a bit of ingenuity and a bunch of clear fish tank tubing. You will need a container that the tubing can attach to on the bottom, and is open at the top, plus a moderately good sized board - say a 1x10" or 12", 2-3' long.
Attach the bottle to one end of the board, and run the tubing up the board in a straight line, on a diagonal, that starts on the end with the bottle about 2-3" up on the board and is an inch or two higher on the other end. Position the bottle and the start of the tube so that you can put a few ounces of water in the bottle before the level is even with or a little above the start of the diagonal. Continue the tube from the high side of the diagonal long enough to connect to the flue.
Mark a zero point near the low end of the diagonal, and then mark very accurately the height of the tube above a line drawn the length of the board through the zero point every 1/2" or so along the tube.
Make a hole or fitting you can attach the tube to in the flue - and some way of connecting the tube that will be low enough temp not to melt it.
Position the board near the boiler with the line through the zero point being dead level.
With the tube NOT attached to the flue, carefully fill the bottle with water until the level in the tube is EXACTLY at the zero point. If you have trouble seeing the water level, try adding a drop or two of food coloring.
Hook the tube to the flue adapter. The draft will pull the water up vertically an amount equal to the negative pressure - this is where the "Inches of water column" term comes from! Since the tube is at an angle, the water will also travel horizontally, in effect spreading out the vertical distance so you can see just how far it went up... Usually you want to see what the reading is with the boiler going full blast, and set your Baro accordingly.
Note that this is a home brew version of a precision instrument - the quality and accuracy of your readings will only be as good as the job you do in construction, and the care with which you use it...
Since I'm not sure just how clear my description above is, I just drew a rough plan for one in QCAD and attaching a JPG of my drawing. The dimensions are not critical, I just included them to give an idea of how it works. Also I accidentally set the scale wrong when setting up the drawing, the green measurements are off by 10, so shift the decimal over one place... On some pro-models of the same idea, I've seen them do a sort of "hockey stick" curve on the angled tube, where the angle gets steeper as the amount of suction goes up, on the theory that you don't need as precise a reading. OTOH, if this doesn't give a precise enough reading, making the tube angle lower will spread the scale out further, with the limit that the lower the angle, the more precisely you will need to make sure the reference line is exactly level. I can't emphasize enough how critical it is to get the level right, especially as the vacuum level being measured gets smaller...
Rather than using a construction level, one can arguably get a more precise leveling action by running a second length of tube in a wide "U" from one end of the board to the other, and filling it part way with water - then measuring the distance of the two water column ends from the zero line (or a second line parallel to it) - if they are the same then the line is level...
Gooserider
