How do you evaluate a chamois (padding in shorts) before buying?
I have bought my last few shorts by direct experimentation at stores, and had the same doubts as yours.

Mostly, what has worked is: you dress the shorts, and stand right up with feet in the normal position. In this position, the shorts should not produce a "bulk" between the thighs. Shorts that produce the bulk tend to be too uncomfortable either while on the bike or while walking.

Also, raising each foot alternately while flexing the knees should not produce a rubbing sensation against the inner thighs, which would mean skin irritation.

I have noticed that the padding should not be flat, that is, it should have a saddle shape. That means you cannot easily fold the short to put it in a drawer, for example, and when you hold the shorts by the waist, the short keeps like "inflated".

At last, the overall compression of the fabric should feel "fine" and specifically the elastic bands around the waist and the thighs must feel "right": a gentle compression, not a tight one.

I think that's it, hope it helps!