How can you tell if a silkworm is male or female?

Caterpillars don't have a sex, they are the sexless larva of insects (although there might be a clue in their DNA - but that would require lab analysis).

But wait: Caterpillars will turn into butterflies, right? Well, just wait until they do, and look for DARK lines around the wings with many white dots (girl) or light lines with little white dots (boy).