You can try a DIY "stethoscope", using either a length of vinyl tube to listen at a precise point, or the wooden dowel that gets pressed against various locations to hear local noises.
A vacuum gauge will indicate overall manifold vacuum, and this can be a good guide.
All carbs have a leak past the throttle shaft, but it is a small and controlled leak. When shafts wear in their bores, is when the leak becomes greater than you really want - and low speed operation is compromised.
Regards, Adam.