You can tell from the underside of the crab’s carapace. Female crabs usually have a more “rounded” pattern on their underside, whereas male crabs tend to have a narrower pattern. Here are a few examples from other crab species to give you a sense:
And an example of Blue Crabs:
The females have a broader, rounder underside since that’s where they hold their clutch of eggs.
Something I am not sure about is whether this pattern holds true for “false” crabs. That would be a neat rabbit hole to explore!
