How to know when a webpage is updated?

You can try ChangeDectection:

ChangeDetection.com provides page change monitoring and notification services to internet users worldwide. Anyone can use our service to monitor any website page for changes. Just fill in the form below, we will create a change log for the page and alert you by email when we detect a change in the page text. We've been doing it since 1999. It's free.

I ended up using NotiPage

NotiPage is a FREE webpages monitoring software. Once you've configured a monitoring alert related to a specific webpage, NotiPage will automatically check this webpage on a periodic basis until all the set criteria are met. At that time, NotiPage will warn you by a visual and audible alert.

Freeware, lightweight, recently updated. Can check every 1 minute.