The first sign is the smell. If you get an unpleasant smell from your squash, it is a good indication that it has gone bad. Apart from this, sometimes, the squashes turn mushy when they are overripe. You may also see some liquid coming out of its rind. And that's another indication as well. There is another way to spot whether the squash has gone bad, i.e., change in color. The outer skin might turn darker in color and may form wrinkles on it. When you see such characteristics, it's time to throw that squash away.