is_s3_generic compares name to .knownS3Generics and
.S3PrimitiveGenerics, then looks at the function body to see if it
calls UseMethod().
is_s3_method builds names of all possible generics for that function
and then checks if any of them actually is a generic.
Usage
is_s3_generic(name, env = parent.frame())
is_s3_method(name, env = parent.frame())