Give them a problem, preferably one associated with the problem domain they'll be working on, and ask them to discuss how they would approach it. You can have them just discuss, pseudo-code or write bits of actual code depending on how confident you are in their skill level

For example, if your organization did conferences, ask them to outline how they would code a secure online registration system. They should be able to cover some of the basics and ask good questions about exactly what needs to be implemented. As you interact, you should be able to determine if they'll be a good fit for your organization and the role you need them to fill.

I'm not a big fan of programming trivia tests and brain teasers. While they can be fun for some people, they can also annoy and/or stress out other people, including people who might just be the best fit for your team. Plus, info on many such tests are readily available online and will encourage cramming for the tests and other tactics that would blunt their viability to gauge programmer ability.