There are many types of tofu. I would say the freshest are the white silken tofu, which are kept in liquid and perish easily. Of these, there are firm, medium, and soft. Soft tofu is good in light, brothy soups or delicate dishes. Medium and hard tofu are good for stir-frying. These are all equally fresh, since the firmness is determined by processing method. However, you know when the tofu has gone bad when it has a sour taste. I'm not sure if tofu does or does not contain gluten, since the best brands, provided in international groceries, do not carry such labeling.