Assuming that the process is not running as root, you can kill -0 $(cat /path/to/file.pid) to see if the current user owns that process, or if it's running at all. Of course, lock the file before checking, and write the correct pid with the same lock.

There is Proc::Daemon which can take a pid file, but I haven't used it, so I don't know if it does the right thing.

Even if the daemon is run from cron, this issue could bite our questioner

The unit is going to be installed in an environment whereby it cannot do anything to alert anyone to its distress if there is an ongoing problem. If it cannot get the time, it cannot function. The only way I will know is by checking if it is reporting in to the server regularly which it is configured to do each time it opens or closes the curtains.

For that reason, I have re-written the above code so that a lockfile is not necessary. Instead, if the time is not set correctly, it logs the problem, increments a counter in the config file and exits. Next time it gets fired up by cron it has another chance to check the time but will reboot if it has been restarted 15 times as half an hour should be plenty of time for the router to come back to life after a power failure is restored.

my @time = localtime;

$control->log("Starting Curtain Controller") if $DEBUG > 1;

# Check Pi has valid time
if ($time[5] + 1900 < 2020) {
    my $restart = $local_conf->key('RESTART COUNT');
    if ($restart > 15) {
        $control->log("Still no valid time - rebooting!");
        $local_conf->key('RESTART_COUNT', '0');
        sleep 2;
        system("reboot");
        exit 0;
    }
    $control->log("No valid time is set - exit $restart");
    $restart++;
    $local_conf->key('RESTART_COUNT', $restart);
    exit 0;
}
$local_conf->key('RESTART_COUNT', '0');
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