The patent agent you want to hire must first practice in a regular patent agency, and it is best not to find a part-time agent.
The professional background of this agent should be close to your invention. For example, if you are a drug inventor, you'd better find an agent with a medical background. If it is a computer invention, find an agent with an electromechanical background.
You can ask the agent what successful cases he has represented, the length of his practice and so on.