In Parameter§

See primary documentation in context for method named

method named(Parameter:D: --> Bool:D)

Returns True if it's a named parameter.

my Signature $sig = :(Str $xBool :$is-named);
say $sig.params[0].named;                          # OUTPUT: «False␤» 
say $sig.params[1].named;                          # OUTPUT: «True␤»