How do you know if milk has lipase or has just gone bad?

If milk has an excess of lipase, it will taste and smell soapy but it is still safe to feed baby and it is very unlikely that baby will notice the difference. If a milk has gone bad, you will definitely know because it will smell rancid, very very bad. Throw this out immediately!

Very bad smell I believe is the common sign of spoiled milk. If it smells different, just throw it away and don't dare give it to the baby even though it still tastes good to prevent tummy illnesses.

The smell of rotten breastmilk is like rotten cheese :) If it smells fishy, that's okay! Had this problem with milk I'd freeze then defrost. It may have been that I ate a lot of fish that day.