Best free tools for find / define your audience?

The cornerstone of your startup journey is being able to define your audience / customer. Sometimes (and very rarely) you can create a whole new marketplace that didn't exist before (Airbnb) but by and large you will have some idea of who your product / service will appealing to and why.

The question is, what tools or research did you undertake to create your customer persona? Is there a suite of free tools you'd recommended? Or is it simply asking friends and potential customers for their feedback?