  Let $K$ be a set of $q^2+2q+1$ points in $PG(4,q)$. We show that if every
3-space meets $K$ in either one, two or three lines, a line and a
non-degenerate conic, or a twisted cubic, then $K$ is a ruled cubic surface.
Moreover, $K$ corresponds via the Bruck-Bose representation to a tangent Baer
subplane of $PG(2,q^2)$. We use this to prove a characterisation in $PG(2,q^2)$
of a set of points $B$ as a tangent Baer subplane by looking at the
intersections of $B$ with Baer-pencils.
