You can know.
If the silkie and the sumatra are both pure, then they will have
one copy of "hookless" and be normally feathered,
white earlobes from the silkie, since that is pretty dominant,
strawberry comb,...(the earlobes will not actually be white due to the combination of yellow skin and dark facial skin pigment (from the sumatra.)
basically one copy for each gene that works on each parent, speaking only to phenotype.
They hens will be really dark. The roosters will be nearly black, all over their whole body. They will have extra toes, or most of them anyways.
Hey what color is the (feathering) of the silkie?
forgot to say, they should have pretty long saddles and tails since the silkies usually dont have any saddle restrictor genes.