How do we know that Todd has forgotten about the dollar?

We would know that Todd has forgotten about the dollar if we observe him not making any effort to obtain the dollar or take any action to recover the dollar. For example, if we ask Todd if he remembers the dollar, he can't recall it or doesn't think it's important, or if a day passes and Todd does not make any effort to obtain the dollar, then we can be sure that he has forgotten about it.