How is the centre of lift estimated for airplane models before they are being mounted in a wind tunnel? They mount the airplane models or parts of an airplane like just a wing, or a rudder in wind tunnels to test their drag and lift, stability, etc.

How do they know where to hang it from or mount it on. How do they estimate the lift center of a new design of a jetliner which has no predecessor to use as a reference.

I am guessing it is a process of trial and error. But there must be an algorithm or method to arrive from an estimate to actual lift center.
you mount it in a convenient way, and then with all the load sensors reporting, you use a spring gauge to apply known forces by hand to the object in known directions and record the load sensor outputs for each. this will tell you how to add and subtract the individual sensor outputs to yield the correct signals (pure lift, drag, pitching moment, etc.).