A module I wrote at work does a rough-approximation of detecting if X11 service is available. It first checks DISPLAY, then it tries to connect a socket to the right port. (Sorry, can't give the whole module.)

use Socket qw(SOCK_DGRAM SOCK_STREAM SOCK_RAW PF_INET inet_aton sockad
+dr_in);
use FileHandle;

# ... other module junk ...

# $spec = display();
#
sub display
{
    return $ENV{DISPLAY} || ':0';
}

# $bool = scan($hostname, $tcpport);
#
sub scan
{
    my $host = shift;
    my $port = shift;

    my $proto = (getprotobyname('tcp'))[2];
    my $fh = new FileHandle();
    my $ip = inet_aton($host);
    return undef
        if not defined $ip;
    return undef
        if not socket($fh, &PF_INET(), &SOCK_STREAM(), $proto);

    my $saddr = sockaddr_in($port, $ip);
    return undef
        if not connect($fh, $saddr);

    close($fh);

    return 1;
}

# $bool = ready();
#
sub ready
{
    my $display = display();
    my ($host, $screen) = split(/:/, $display);
    $host ||= 'localhost';

    return scan($host, 6000 + int($screen));
}
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There are other situations where the port in question is not so easily computed; these usually relate to tunneling of some sort, such as with (ssh -X) commands. Extend as desired.