It depends completely on the job. But essentially you want to find out a few things:
- can the person perform the work required, with the training that will be provided?
- does the person have a work ethic that will get them to work on time, and keep them working productively throughout the day with a normal amount of oversight?
- is the person going to fit in with the company culture and other employees?
There's a lot more I'd personally want to find out (do they actually want the job, or is this just a stepping stone or gap filler? What do they know about the company, it's products and services and its customers?) but I think the above three are pretty universal regardless of the position.
Rather than asking questions directly, ask for examples of times they would have fulfilled what the question intends to find out.
For instance, if its a waitstaff position, rather than asking "can you handle waiting 4-5 tables during evening rush?" Ask "tell me about a time when you felt overwhelmed waiting on tables." Then proceed to ask follow up questions about that experience. Did they seek help? We're their customers happy? How did they manage to meet everyone's expectations while being overwhelmed? It would be suspect if they've never felt overwhelmed, or didn't want to admit to feeling that way, for instance. One of my favorites is "tell me about a time you made a mistake." Then follow up with questions about the mistake, the impact it had on others, what they tried to do to fix it, etc. how they handle themselves under pressure and when the have failed can tell you a lot about their work ethic.
Open ended questions generally tell you more, and more interesting anyway, than simple yes/no questions. You don't want to be bored, and you don't want them to be bored.
Ask them if they have any questions. Depending on the position and job, it can be telling to find out whether they think they know everything, or have particularly interesting questions about the position, company, or people they'd be working with.