  The tilting correspondence is a fundamental property of perfectoid fields. In
this note, we show that the tilting construction can also be used to detect
perfectoid fields among nonarchimedean fields. In particular, for $K$ a
complete subfield of $\mathbb{C}_p$ (a completed algebraic closure of
$\mathbb{Q}_p$), $K$ is perfectoid if and only if its tilt is not algebraic
over $\mathbb{F}_p$. We also include some conjectures on APF (arithmetically
profinite) extensions, perfectoid fields, and their relations.
