Get some referrals from your neighbors and interview the agents who have done great a job for those neighbors. There are common real estate interview questions every full time, in the trench agent, should be able to answer at the drop of a hat. You'll know it when you find it.
If you want a list of the right questions to ask, just get in touch =)
My advice is to find a buyer's agent who is working in the local area where you are looking to buy. (If you have not signed up with anyone.) Check out her web site and profile to see if she has ABR (accredited buyer's agent) after her name and some experience in the area. Just call her and chat to see if it's a good fit for you. Good luck!
If you do not have a buyers agreement signed with your friend then you are free to use another agent to represent your best interests during the negotiations. Its best to use an agent other than the listing agent because once the listing agent represents both parties they are no longer allowed to give advice/suggestions on what to do. If you have not already written up an offer I would be happy to represent you. There is no fee for a buyers agent and it just so happens I am also a Certified Negotiation Specialist so you know you would be getting the best price on this home. I also would attend the inspection with you and if any problems are detected I will go back to the negotiation table to for you. Please let me know if you have any questions I'm happy to help = )
If you work with the listing agent, you should be given written notice of either dual agency or no agency regarding that property. This is the law! The listing agent is under contract to represent the seller not you!! You should have a written representation agreement with you friend which spells out their duties to you. If you don't have this you are free to find another agent who will rfepresent you properly.