You know you are senile when
Let's see how funny we can get with this one:
1) You know you are senile when you go looking for your eyeglasses only to realize they are on your face;
2) You get up to get something and can't recall what it is so you go back sit down realize what you went for then you go to get it and can't recall what you went for, you do this cycle five times in a row. One is normal five times in a row is senile.
3) You glance about your office convinced that you are at home. Or you look about your home absolutely sure that you are at work.