So... if you take the tops of the SU carbs (dome, piston and needle) and look down from the top...
You should see the "bridge" the raised section in the bore of the carb, with a circular hole in the middle. Inside the hole, flush with the bridge you should see the top of the "jet bearing" (the bit the jet slides up and down in) and inside that you should see the top of the jet itself.
There should be a measurable height difference between the jet bearings (flush with the bridge) and the jet:
I've used the "tail" of a vernier caliper in the past to check it - it takes a bit of care AND you need to make sure the choke is fully home and the jets are fully raised (not by you touching them! By the action of the return mechanism) before measuring.
On this stolen image you can see the bridge and jet bearing, but the jet is not fitted: