There are tests for that, but republicans would fight tooth and nail to not perform those, so we won't know for sure. (Other than republicans being afraid of the results being a sign both sides of the isle know very well what's happening.)
@NerdyPotato I really hope not. I really don't think it matters what anyone thinks of Trump. If I were an American, I would have voted for Trump in the last Presidential Election, but if he showed signs of dementia, I would want him step down, or be made to do so. That didn't happen with Biden but it should have. If it happened to Trump, the Republicans need to show they are better than the Democrats by not trying to cover it up.