You’ll most likely find the guest post guidelines:

Linked to from the blog’s About, Contact or Guest Post submission page

Linked to from the sidebar or the footer of the blog

If they’re not in either of those places, search for “guest post guidelines”, “write for us”, “submit a post”, or “guest posting” on the site. To do this site search, you can type the phrase plus site:[nameoftargetblog.com] into Google, e.g. “guest post guidelines site:ryrob.com” and get more accurate results.

Guest post guidelines vary from blog to blog, but most will cover: