Ideally something professional that shows they actually understand the work involved and actually have some experience freelancing: "this will mostly consist of frontend code, and I'll probably do it in AngularJS because ... it will probably take 3-6 weeks and cost around $x because ... here's a contract with the details, one third payment upfront, next third on X milestone, final third at completion, this product will be yours as a work for hire, etc."
Look for someone who is willing to question you throughout the project to get at the requirements as well. Lots of freelance developers, especially budget ones, tend to implement only what's *exactly* spelled out for them. Higher quality ones will push back and probe a little, because did you *really* mean that the button should go exactly here, or do you want it to go in the standard place that this button normally goes in other apps? Lots of my clients came to me after small frustrations with lower cost freelancers, not always because they did a poor job on the coding (though they usually did), but because they would do things like default state dropdowns to Ohio for a website based out of New York. Those little things aggrieved them enough to pay my significantly higher rate :)
As a rule, you probably don't want to hire anyone who simply says "I understand the work, and ready to start". They probably don't understand the work, at least not to the level of detail that they can deliver client-happiness. Really decoding requirements takes a two-way discussion; communication is always critical to the success of a project.