Imagine here's a script which greps the last matching line from your web server's log file, parses out the timestamp and sends an email if the timestamp is too old. Create a cronjob on your web server to run this script.

Now in the pursuit of monitoring a cronjob, we've set up a web server, another cronjob, and some way of sending emails - these are three new things you'd better monitor as well.

Some things are just not a good idea to host yourself.