Recommendations for a Scrum Master joining to an established scrum team

Your first priority should be learning how the team operates.  If they are an established team and have been successful in delivering value, you should not expect that there are changes needed. You need to establish a relationship with the Developers and Product Owner.  Learn how they communicate, how their process works.  Ask them if they feel there is any needs for improvement.   A Scrum Master is not part of the team to make changes they think is necessary.  They are there to help a team facilitate change that the team feels is needed.  

If they have been together, following Scrum principles for a couple of years, this could be the easiest job you have ever had. I have been part of mature Scrum teams as Scrum Master that never or very seldom needed me to help. I actually challenge myself to become unneeded.  That shows I am fulfilling my duty to the team of helping them to understand and appreciate the Scrum framework while helping them to become self-organizing and self-managing. 

I'd like to hear your recommendations and suggestions for a scrum master joining to an already established scrum team following scrum principles for a couple of years. What are the areas that you think would be important to focus on first? Is there any recipe or roadmap for a scrum master to follow for such circumstances?

First, check your assumptions carefully. How do you know that they have been following Scrum principles? What has lead you to this opinion?

Great feedback. Thanks everyone. I'm new to this organization as well so would be really good to check the assumptions about their Scrum maturity first as it has been told me that the team has been following Scrum for the last couple of years.