How do I best interview sales people for a startup?

On interview,create simulation of selling some product by candidate,and you will see everything.

An interview is helpful but you should spend most of your effort referencing the candidates previous sales experience.  The nice thing about hiring a salesperson is that you can look at concrete prior performance as a good guide to their future potential. First, you want to make sure that they have sold to the customers that you're company sells to. Do they have a Rolodex that will get them in the door with your target customers?Second, you want to know about their prior sales performance. How did they rank in their prior companies?  What sales awards did they win? Did they consistently meet/beat their quota?  Get references of their sales managers at prior companies to call for a reference.

I agree with Mr. Zaid answer. Thank you 

First of, get to know there background, discuss there GP'S achieve, en there monthly figures done, you will get a vibe, on that person. Then let him/her do a role play, greeting customer, discuss what the needs are, and let them sell something to you. You as the interviewer, must ask strong product questions, and see how he/she handle it. Remember, you get lots of different customers, so, that sales person must pick up fast, is the customer looking for something specific, and or, is she open the chance his /her mind on selling up. Always look at the persons body language, and if he/she eye contact. This will help a lot in a pointing strong sales persons.