First is to get recommendations from friends. Normally, your friends can give you an assessment of their recommendations without feeling blunt or awkward about it. Also, based from my own experience, most of them are reliable. I'm not saying every one of them, but a good fraction. You can't generally assume that you can get good recommendations from them all the time because they are your friends. You still have to use your gut. If you can't get any recommendation, then you can go to the most reputable property management company you know.
This company has to be from your area and not from somewhere else. That way, they can gauge the proper valuation for your properties, and know what it is that renters are looking for. They should also be able to structure a design that suitable to your needs. Moreover, ask them for 2 or 3 references which you can call to get an overview of how they work from a third party's point of view. They could be telling this and that about their services only to find out that they actually didn't do as good in their previous projects as they are blabbering to you.
In addition, don't forget how they value the tenants. It is important for a management company to know how to deal with renters. They are the representation of the landlord, and have to embody the landlord's ideals. Thus, they are responsible in maintaining a good landlord-tenant relationship. As I've already said up there, they assume all routines of the landlord.
Once you've chosen a company, set up a meeting with them. Getting to know the people you work with is essential to building a strong and lasting relationship. It will also get them comfortable to interact with other team members.
A competitive book keeping services is vital for a management company as well in order to keep track of your business' progress through an assessment of the finances. And the property manager they provide has to have a passion for the industry and serving people. After all, tenants are the lifeblood of your business so they have to be well-taken care of. I don't know how many times I've already said this but it's better to take care of your existing renters than to look for new ones on scratch.