Look at their website case studies, as well as what other clients say about them on Google My Business reviews, not the reviews or testimonials found on their website, as these can be manipulated easily.

Ask them what strategy they would take for your project, and see if it makes logical sense to you. It should not sound complicated, because it’s not, if they are not being transparent with the work they will do for you, then take it as a red flag.

More importantly, make sure, they are not a linkbuilding farm, but look for an agency at also helps you develop appropriate content and improve the one already on your website.