The easiest way, is to do a HEAD on the URL in question and see what the returned headers tell you. So, something like:

use LWP::Simple qw/head/;

for('http://perlmonks.org/index.pl?displaytype=xml;node_id=484407',
    'http://perlmonks.org/index.pl?node_id=484407') 
{
   my ($type, $length, $modified_time, $expires, $server) = head($_);
   print "File is: $type\n";
}
[download]