How to identify if the frozen breastmilk has turn bad? if there is excess lipase, the milk also has a sour smell, is that right?

Spoilt Breastmilk smells really rancid. It would also help to know how spoilt Breastmilk taste/smell like so that you know how to identify it in the future. Leave a tablespoon of BM on the countertop overnight and do a taste/smell test the next morning. And if you are unsure, my experience is that Bf babies are smart, they will reject spoilt milk