sub foo :method {
  my $self = shift;

  # ...

  return;
}

sub bar {
  # ...

  return;
}

use attributes;
sub is_a_method {
  scalar grep $_ eq 'method', attributes::get($_[0]);
}

for ([ foo => \&foo ], [ bar => \&bar ]) {
  print "$_->[0] is a " . (is_a_method($_->[1]) ? 'method' : 'function') . "\n";
}

__END__
foo is a method
bar is a function