Verifying information is not always easy or accomplishable. You should attempt to reach out and verify what you can. If a shady SEO business was going to scam, then I am sure this is where they would start, with testimonials. I suggest verifying what you can, but don't get overwhelmed with referrals because it may be the shady SEO business giving self praise. I actually have some good verification tips, but that's another article all within itself. This Site