Answer honestly--that's all you can do, or are expected to do. That means that if you don't know at all, say "I don't know" or "I don't remember." If you have a weak recollection, you can state that you *think* it was such and such, but are not sure; for example, in a car accident case, if asked the make and model of the car that hit you, and you are prettry sure it was a Toyota but not which one, you could say something like "I think it was a Toyota, because I though I saw the Toyota badge or symbol on the car, but I did not recognize the model and have no idea what model it was."