Find an agent that is experienced. Not only in the real estate side, but the mortgage lending side. Really, if they are experienced & cross-trained in every level of the transaction, they have the ability to protect and represent you well.
Do you connect with them...are they honest/polite? (polite test= tell them your most likely going to work with another realtor and see how the react). If they are easy to open up to and talk with, you will mitigate issues, surprises, and buyers remorse a great deal. Nothing makes a deal go smoother than open communication.
Find an agent that can guarantee you fiduciary representation, looking out for your interests above everyone else, without exception. Most forms of dual agency are not to your benefit.
In most states there are laws regarding procurring cause. Meaning, if you have an agent show you a home then they were the one who initiated your first exposure to the home and therefor they have the procurring cause to sell it to you. However, if you just visited an open house and this agent was there, this will probably not apply. If you enter into a buyer/broker agreement with another agent after the fact, then the new agent is your rep and the agreement takes precident. Your question is lacking some detail, so it''s hard to answer for sure. You should really consider hiring a buyer's representative to represent you in any case. I hope this helps. Good luck!