The function canonicalize-dspec checks that dspec is syntactically correct and returns its canonical form if dspec is valid. Otherwise canonicalize-dspec returns nil.
CL-USER 12 > (dspec:canonicalize-dspec 'foo)
(FUNCTION FOO)
CL-USER 13 > (dspec:canonicalize-dspec '(defmethod bar (list t)))
(METHOD BAR (LIST T))