A successful digital marketing company is one that is able to consistently generate leads, conversions, and sales for its clients. Look for companies that have a proven track record with successful campaigns in the past and are able to provide analytics and other data points to show how their efforts have translated into success. Also, look for companies that use cutting-edge technology and innovative strategies to ensure their digital marketing campaigns are effective and up-to-date. Ask for testimonials from clients they have worked with and make sure they follow best practices such as A/B testing and data-driven strategies.