Always look out for agencies that have experience with search engine marketing strategies and have a proven system that works. You should also consider these:
1. Avoid anyone who speaks about it abstractly because SEO is not magic.
2. Be clear about your goals and seek a firm that can achieve them.
3. Don't just pass by a Google search for "best SEO" lists.
4. You must employ an SEO firm with metrics that work for you.
5. Consider an SEO firm that is effective at communicating.