From type/Attribute

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method has_accessor(Attribute:D: --> Bool:D)

Returns True if the attribute has a public accessor method.

class Container {
    has $!private;
    has $.public;
}
my $private = Container.^attributes(:local)[0];
my $public = Container.^attributes(:local)[1];
say $private.has_accessor; # OUTPUT: «False␤» 
say $public.has_accessor;  # OUTPUT: «True␤»