> (functionp #'+) == (functionp (symbol-function '+))
'+ ならいいけど、誤解を招かないとも限らないので一応

* (flet ((f ())) (functionp #'f))
T

* (flet ((f ())) (funcitonp (symbol-function 'f)))
Error in KERNEL:%COERCE-TO-FUNCTION: the function F is undefined.
[Condition of type UNDEFINED-FUNCTION]