A yeasty smell is fine. It's the wild yeasts that populate it and make it ferment.
If it's bad it has a noticeable bad smell. Not yeasty or fermenty, but gross.
As an aside, it is possible for a batch to go bad. Because of the high heat here in FL I usually start a fresh batch about every week. Otherwise things can go icky... maybe after two or three weeks, hard to tell. So I cut my chanced of having a batch go bad that I'm planning on feeding that day by heading it off and starting up new batches fairly regularly.
"Icky" is subjective. A deep ferment in hot weather smells like vomit...but that doesn't mean it went bad. Left for a couple of weeks, never stirred and never refreshed, then yeah...you will use up all the available feed for your scoby and they may start to die and let nasties grow in the feed. But, a deep fermented smell does not mean the feed went bad. Mine gets that smell within 3 days of refreshing the feed while we have hot weather...smells icky but is NOT icky, still healthy to feed out and just fine for their consumption.
Good idea to rebatch more frequently in the hot weather to avoid all that, so you are on the right track. SCOBY accelerates in metabolism in the high heat and can use up available feed for them quicker....I usually rebatch once a week in summer also.