As an alternative to HTTP/HTTPS requests, you can "ping" checks by sending email messages to special email addresses.
Consider a cron job that runs weekly and sends weekly newsletters to a list of email addresses. You have already set up a check to get alerted when your cron job fails to run. But what you ultimately want to check is if your emails are getting sent and delivered.
The solution: set up another check, and add its email address to your list of recipient email addresses. Set its Period to 1 week. As long as your weekly email script runs correctly, and there are no email delivery issues, cron.roflcopter.fr will regularly receive an email, and the check will stay up.