"Ale je vdaná."

Translation: But she is married.

[snikta]

how do I know from the sentence that it is a she and not a he?

[ValaCZE]

In czech? When is it she , you use vdaná;when is it he, you use ženatý

[igamenir]

As ValaCZE says, but the grammar difference can be simply seen in the -á for her and -ý for him. ;-)