You need to know some important stuff about them here are few stuff you need to know:
1- Their skill-set and what skills they have that you need in your project
2- What apps can they use because there are different purposes for different apps
3- You need to see their portfolio to know if this is really the person you need to work with
4- Know their price range don't go with people that are too low because that sometimes indicates the quality of their work and don't go too high because high price doesn't always mean high quality
*Bonus tip: Try communicating with them and give them a quick brief about what you need and see if they will be able to do it