I juiced some apples, took off all the sediments from the top, placed those in my brew bottle, and poured the fresh made apple juice in. (I made it as sloppy as I am explaining it.) I then poked a hole in a balloon and placed it atop my bottle. It has been sitting there since Sunday, the balloon expanded after the first day, and when I look i can see sediments rising and falling. How will I know if my batch is bad?
The microbes that are present in the juice (from the skins of the apple and on the press itself) will eventually replicate and start to metabolize the sugars in the juice.
If, by "bad", you mean "poisonous or harmful to you", then the answer is that it will not be harmful or bad, except for the harmful effects of alcohol. The acids and alcohol created by a fermentation do not create not a hospitable environment (like the human body) for pathogenic microbes, fungi, spores, etc.
If, by "bad", you mean "My friends and I do not want to drink it", then the way you are fermenting the juice is a gamble. You are better off buying a $2-3 satchet of wine yeast and adding that to the juice. It will likely out-compete the microbes for the food (sugar/fructose) and produce a relatively clean-tasting cider.
Either way, you should taste the juice in a few weeks and see how it tastes.
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