Actually, you don't. If you use FreeBMD, then search for the husband, find him, and see the marriage registration, say
Kingsbridge 5b 511
and then click on the '511', it will show you all the names on that page, and from 1912 it shows who married whom. (You could always cross-check with a census.)
If you know any of the children, you can look them up on GRO Services, and it should have the mother's maiden name right from 1837.