If you're not sure how to tell if tofu is bad, there are a few things you can do to check. First, take a look at the expiration date on the package. If it's expired, it's probably not good anymore. Second, smell the tofu. If it smells sour or off, it's probably bad. Third, touch the tofu. If it feels slimy or sticky, it's probably bad. Finally, taste the tofu. If it tastes sour or off, it shouldn't be eaten.