  We investigate Andreev reflection in two-dimensional heterojunctions formed
by a superconductor in contact with a topological insulator ribbon either
possessing or breaking time-reversal symmetry. Both classes of topological
insulators exhibit perfect Andreev reflection, which is robust against
disorder. This is assigned to topologically protected edge states. In the
time-reversal symmetric case we show that doping one of the ribbon edges with
magnetic impurities suppresses one Andreev channel, while no such suppression
is seen in the broken symmetry situation. Based on this observation we suggest
a tabletop transport experiment able to distinguish between the two types of
topological insulators, which does not involve the direct measurement of the
material band structure.
