http://www.site24x7.com/ http://www.site24x7.com/ has a cert monitor that does exactly what you want and more. If you have 10 domains it will run you $10/month.

http://www.site24x7.com/ http://www.site24x7.com/ has a cert monitor that does exactly what you want and more. If you have 10 domains it will run you $10/month. (Although you probably also want an uptime checker, so really it's more like 5 domains on the $10/month plan.)