Call a contract organization and bring in a few people for a 6 month contract. At the end of the contract offer jobs to the good ones.
6 Months is only because that is what contract organizations want - you can buy the good people out sooner, but they need 6 months from people to make their budgets work. 6 months is plenty of time to ensure you know if someone is bad or just slow to get productive, but not too long to pay someone who is not worth it.