You can ask for references, past clients who were happy with their level of service. You may also want to get something in writing that says if they don't provided a certain minimum service they will allow you to cancel your contrat. But expect to be asked to live up to your end of the bargain as well. Today's market can be a challenge to both buyers, sellers and real estate professionals! No amount of marketing will sell a home that is over priced. And today's buyers expect a home to either be in move-in condition, and I mean complete with updated kitchems, baths,etc, or to purchase the home at a discount. I think it is absolutely fair to ask your realtor to provide some kind of assurance they will do what they say they will do, but it works both ways, sellers need to heed their agents' advice as well, especially when it comes to price and preparing the home to look its absolute best while it is being marketed.
If you will work with your agent as a team, I think you will be fine.