You can almost always use either ref or Scalar::Util's reftype, as mentioned by the other monks. The only time that approach would fail is when the object being tested acts like a hash using the magic of overload. In those rare cases the only solution I know is to try and use it like a hash.

{
 package fakehash;
 use overload "%{}" => sub { return {1 => 2} };
 sub new {my $c; bless \$c};
}

use strict;
use warnings;
use Scalar::Util qw( reftype );

my $fake = fakehash->new;

printf "fake is ref %s\n", ref $fake;
printf "fake is reftype %s\n", reftype $fake;

# Try to use $fake as a hashref. 
# The return value of eval should be 1 if the first
# statement is successful, as 1 is the last statement executed
# in the block
# The assignment to (undef) is to avoid this warning:
#  "Useless use of a variable in void context"

if ( eval {(undef) = %$fake; 1}) {
    print "fake is pretending to be a hashref\n";
}
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