You can tell when a zucchini has gone bad when you cut into the inner flesh of the zucchini it may be stringy and that stringy flesh is a sign that the zucchini is bad and should be thrown out.
On the outer part of a bad zucchini the skin will look dull and probably wrinkled a bit and it may also have rotten spots or mold on it and if it looks decayed or is starting to smell bad then the zucchini should be thrown out.
If you refrigerate zucchini it will last up to 4 to 5 days if properly stored in the refrigerator and after that time the zucchini is likely bad and should be thrown out.