I used to work in our company's recruitment dept.
First the obvious stuff like - neatly presented, on time, eye contact, pleasant handshake.
We always asked questions about when they had had a problem at work or something had gone wrong and how they handled it. People who say they've never made a mistake or had a problem are liars or egotists or blame shifters.
Always ask why they want the job. if there answers are about what the job can do for them, not what they can bring to the job it's a warning sign. So if they say: 'I need the money,' even in a jokey way, it's a red light. But if they say: 'I really enjoy customer contact and there's seems to be more of it in this post than in my current place of work,' then you know it's about what they can bring to the job. (Having said that, clearly mums with small kids who want PT work that fits in with school hours will be looking specifically for that and will say so. They might be far more reliable because those jobs are gold dust.
We asked stuff like: what is your dream job? If it had no connection to the business then again, they are less likely to be genuinely engaged.
Listen out for how often they refer to family and problems, how often they whinge, even jokily. These are warning signs that they are not professional. They should be positive in their language and fully engaged with the job on offer.
Ask if they have any other questions. If their questions are about your organisation they are interested, especially if they ask what training they'll be given. It suggests a keenness to do the job well. If they are about holiday pay and TOIL then you know what their priorities are.
Finally, always check both references, and to make sure they're not just mates (it happens) ask tough questions or for a written reference, which should be professional and give dates of work which match their job history on their CV.