1. There are only three toothbrushes in the family toothbrush holder but four people in the family. (However, this phenomenon can also be explained if you have a spouse with OCD tendencies, who uses toothbrushes to clean mildew on bathtub grout while sitting on the can. Lars, I may or may not be talking about you.)
2. When you send him up to bed, say don’t forget to brush! and go up there twenty second later to check, his lights are out and he’s already snoring.
3. The dentist's disclosing tablets make him look like he just bolted one of Aunt Ida's blueberry pies. Every. Single. Tooth.
4. His breath smells like your hundred-year-old grandmother’s does.
5. He does not look you in the eye when he assures you he did the deed (this becomes a bit less reliable if he’s on the autism spectrum).
6. He asks for the meaning of “tooth sweaters.”
Dr. Watson, I think we can deduce that Benjy has not been brushing.