Does comparing a pointer that has been free'd invoke UB?

Using a value of a pointer after the object it is pointing to have reached it's lifetime end is indeterminate as stated in the C11 Standard draft 6.2.4p2 (Storage durations of objects) (the emphasis is mine):

The lifetime of an object is the portion of program execution during which storage is guaranteed to be reserved for it. An object exists, has a constant address, and retains its last-stored value throughout its lifetime. If an object is referred to outside of its lifetime, the behavior is undefined. The value of a pointer becomes indeterminate when the object it points to (or just past) reaches the end of its lifetime.

And using it's value (just for anything) is an explicit undefined behavior as stated in Annex J.2(Undefined behavior):

The behavior is undefined in the following circumstances: [...] The value of a pointer to an object whose lifetime has ended is used (6.2.4).

Yes, using a pointer value that has been freed for anything -- even a seemingly-innocuous comparison -- is, strictly speaking, undefined behavior. It's unlikely to cause any actual problems in practice, but I'd say it's worth avoiding.

See also the C FAQ list, question 7.21.