I did some wild ferments few years ago. The main criteria is if it smells and tastes good.

• lactic smell and taste (like yogurt or cheese) is fine but for some too acidic

• alcohol don't smell that strong, it is usually just a hint

• fruity and funky notes are good

• vinegar smell is bad, it means you made vinegar

• bad smell and taste like bitter, chemicals or rotting this can be dangerous, but it's disgusting so you don't drink it

• ...

Try it and you will see.

I've been using wild yeast for various beers, wines, etc since 2012

I've made tepache a few times and it will go alcoholic if you leave it too long. if the abv is 2-3+% then it's likely safe to consume (if you're overly concerned you can also check to confirm that the pH is below 4). However there probably won't be any residual sugar and it may not taste all that great.