Do a pstree for each screen process.

screen_pstree.pl

#!/usr/bin/env perl

use warnings;
use strict;

opendir my $dir, "/var/run/screen/S-$ENV{USER}/";
foreach my $file (readdir $dir) {
    my ($pid) = ($file =~ /^(\d+)\./);
    next unless $pid;
    print `pstree -p $pid`;
}
closedir $dir;