How does one determine if a blog is a valid or reliable source? Most importantly, by reading and following links. I often skim over a blog's last two months of archives if I'm trying to get a feel for it. I think you can get a better sense of a person by reading them at length than you can by looking at their credentials.
Follow the links, both in the posts themselves and the blogroll in the sidebar. If a blog approvingly links to other sources you know to be unreliable, you should consider the linking blog to be unreliable.
Beyond that, you need to know the difference between primary, secondary and tertiary sources, which isn't unique to the internet. Blogs can be any of the three, however, which is one of the things that makes them a tricky case.
The rest of the article is worth your time, as well as the time of your students, if you ever ask them to do research on the Internet.