If necessary, ask for references or see if they have earned the Accredited Buyer Representative designation. The listing agent owes their loyalty to the seller and will be looking out for the seller's best interest only - you will be on your own. All they will do is write up the offer to purchase at the amount you specify. The listing agent, by law, must tell the seller if you indicate you are willing to pay more than what you offered for the house. Remember - they work for the seller, not the buyer.
Most agents work with both buyers and sellers.
Just make sure your listing agent is just as talented in Buyer Representation or has an assistant who is. A small number of Buyer's agents do not accept listings. Instead, they will refer you to someone within their office who can serve you better.