Ask A LOT of questions:
* Ask about their fee schedule.
* Do they charge a flat fee (i.e. $50/door) plus 1/2 the first month's rent?
* What is their tenant screening system - what are their minimum requirements, what questions do they ask, what form(s) do they use, how much do they charge as an application fee?
* [Don't ask - but you want to know if they will put 'anyone' in your unit, evict them, and make their profits off 'churn'. You want a PM that's interested in keeping tenants LONG TERM, and not 'churn'... but no-one will admit to causing churn because they place bad tenants.]
* Do they charge for maintenance calls? (either you OR the tenants)
* Do they do regular checks on the property?
* Do they have a regular maintenance schedule?
* Do they have a system in place for tenants to report maintenance issues before they become emergencies (i.e. leaky faucets)?
* How many evictions have they done in the past 3 months?
That should get you started. Good luck!