  Let $K$ be a convex body in $\mathbb{R} ^d$, with $d = 2,3$. We determine
sharp sufficient conditions for a set $E$ composed of $1$, $2$, or $3$ points
of ${\rm bd}K$, to contain at least one endpoint of a diameter of $K$ (for
$d=2,3$). We extend this also to convex surfaces, with their intrinsic metric.
Our conditions are upper bounds on the sum of the complete angles at the points
in $E$. We also show that such criteria do not exist for $n\geq 4$ points.
