Once milk is thawed you must use in 24 hrs do not refreeze. If milk is warmed you must use, you can’t refrigerate and reuse. If a bottle isn’t finished and there’s still milk in the bottle, you’re supposed to toss it. I usually just refrigerate and reuse the same day, otherwise I toss.
If you are unsure if milk is spoiled or not you can smell it, it will spell like spoiled milk (sour smell) or you can taste it. If it’s not sweet, but tangy and foul tasting it’s bad. Still unsure, offer it to your baby. If it’s bad they won’t eat it — mother nature is pretty smart that way! Twice when my husband tried to feed Archer a bottle he rejected them, we didn’t understand why and only when we tasted the milk realized it had gone bad. We’re not sure how it went bad… but it did and Archer knew better than us.