Not real sure externally how to tell except for the data plate. Internally, the -21 has a much more durable hot section. -20 has a single fuel manifold, I think that the -21 might have a multiple fuel manifold. Haven't really seen a -20 for about 12 yrs. Operated a DHC-6-100 for a while, real dog. Converted it to -27s, then it was a real good airplane.