How to check if a function reference is valid or not?

sub abc {
print 'abc';
}

my $f1 = \&abc;
my $f2 = \&abcee;

print $f1; # return CODE(0x..)
print $f2; # also return CODE(0x..)

$f1->();
$f2->();

How do I know if $f2 is a valid function reference, without actual
calling it?

Thanks.

.