I know this is old, but for anyone reading this in the future, here are my suggestions.
First I'd make sure you're clear on what services you are getting for the price.
I'd ask about their processes they go through for leasing your property and then for ongoing management. How do they list the property, where do they list it, what is their criteria for screening and selecting tenants? How do they determine rental prices in the area? For ongoing, how do they collect rent, handle maintenance calls, deal with problem tenants? Do they provide accounting and bookkeeping, monthly statements, direct deposits to your account?
Ask for specific examples of how something went wrong in the past and what they did to fix it. Ask for references and follow up with those references (Take what they're saying with a grain of salt, they won't likely give you a reference unless they know it will be a positive one). Check google and yelp reviews.
Pay attention to how they respond. Do they seem honest? Are they friendly and polite? There's no reason to deal with someone who is being rude. Also, did they respond quickly? They likely aren't going to try harder AFTER they have your business.
Hopefully this helps, if anyone has a PM that they are considering but have questions about something they said that doesn't seem right, feel free to reach out and I'll give my opinion.