One of the starting points might be to look at how well-published/highly-regarded the faculty who specialize in that specialty are at the school. And then see if that school has designated programs specifically for that specialty, like a laboratory (for behavioral econ, etc.), academic concentrations, and maybe also infer from the website as an additional reference point to see how the school presents that specialty (one of their strongest, not mentioned, or somewhere in between). And of course there's word of mouth, e.g., learning from people currently/previously affiliated with the specialty at the school, etc.