It's not easy. That's why Wasabi annonymity score is extremely pessimistic. When you see a Wasabi coinjoin output you don't know whether that is a self-payment, a real payment to someone in a coinjoin, a shared utxo or what.
Discussion
If I can't measure the entropy gained, how is it not "trust me bro" privacy?
Because you can review the on chain footprint of the transactions yourself?
its true that if we make certain assumptions
like, all other participants are honest actors
and that no outputs are sent to resused addresses
we could measure entropy gained looking at the chain
but really what a user needs is to know what privacy they gain BEFORE they use the tool