Reference is the best way to do.
For example, I am very happy with the management company for our condos. So I call them, they told me that they don't manage single units, then I ask you have any recommendations, and they give me one or two names. I call and telephone interview what their service area and scope is, how much they charge and so forth, and see how comfortable I am with them.
You can use your neighbor who has investment properties, your boss or coworker who has investment properties, your real estate agent also can give you some recommendations (be a little careful with this one unless you trust your agent 100%).
Good luck