Pay Per Lead.
Most agencies will charge as a percentage of spend which will incentivize them to spend more of your dough. That doesn't work well for you as a business owner.
If instead you are charged per lead, then an increase in spend means an increase in leads and more customers.
If you run a limousine business for example and the value per enquiry is $20, you can ask the company how much they would charge on a per lead basis. If they say $10, then you should try it out and see how well they convert.
This is the best way to make sure that you get a positive ROI on your spend and it reduces your risk as well as the amount of work you have to do screening for people that have no idea what they're doing. If they can deliver solid leads to you at a decent cost, then they know what they're doing and you get a win-win situation.
Good luck!