Great question! For me due diligence boils down to:

• Do I enjoy this person’s blog or other free content? Do I get value out of it? The best courses I’ve found is by people whose blogs I love reading and actively learn from.
• I also download and enroll in any free content they may have (email course / ebooks etc) to get a feel for their teaching methods
• I google around for any reviews to see what people say about it.
• Do they offer a money back guarantee? Also does money-back come with any conditions? If it’s 30 days it should be enough time to skim through the course to see if this is something you want to continue pursuing.

I’m pretty there’s other ways to do it, but so far this process seems to work for me :)