Ruijie Yu via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org> writes:

> In that case, I had my semi-parallel second proposal: provide a command
> that checks for a group of variables and see if types of all variables
> within the group (or its subgroups) are correct.

Regarding the second proposal: I have quickly hacked together a
functional example of type checking, see the attachment.  If we think
this should be integrated into Emacs, I can make some minor edits and
propose a patch.  In that case, I would probably put the functions
somewhere in cus-edit.el or custom.el.

Note that at the moment it depends on the internal function
`custom--standard-value', which is a one-line function accessing a
property of the symbol.