That's a great question that I wish more people would ask of anyone who sells anything related to SEO and marketing.
I'm a developer first and foremost.
I've learned advanced SEO as part getting traffic to my own business and to help customers, but I'm not an SEO consultant or play one on TV.
The SEO consultants I talk to and get advice from are 100% above-the-board (called white-hat in SEO jargon). They are people who have gotten the results by staying on the good side of Google and providing value to their website visitors.
Even then, I still double check their advice to make sure it's safe.
But anyone can say this so, I'd recommend taking at a look at the reviews below. All of them are from actual paying customers, no one there has gotten a special deal or discount for providing a review (In fact, it's against Shopify's policies too).