How to obtain customize :type for an option at runtime?

[Drew Adams]

How can I determine, for a given user option, what its Customize :type is?

For example, function `user-variable-p' tests whether a symbol is a user
option, but what if I want to test whether a variable is a toggle (binary)
option?

Although not all options that can act as toggles are defined with a simple
":type 'boolean" in Customize (far from it, and there are good reasons for
that), obtaining such :type info would be a start.