How to find the name of the man page of something?

You're looking for apropos; on my system apropos luks points me to cryptsetup(8), luksformat(8) and a number of other relevant manpages.

apropos, which is equivalent to man -k, looks in the installed manpages' names and descriptions for the search text given on its command line. The search text can include regular expressions or shell-style globs (with apropos, using the -r or -w options; -r is the default).

man -K allows searching in all the contents of all the installed manpages. This takes longer than apropos or man -k.

(Thanks to Stephen Harris and clusterdude for the extra clarification.)

Try using apropos, it will search in the titles / descriptions of manual pages for a certain string :

apropos luks