Ask HN: Efficient ways of gauging a candidate's English proficiency?

We've found a bit of a bug in our hiring process. Our latest addition is performing pretty well, but their English skills are seriously lacking. Especially in writing. This is a bit of a problem considering one of their tasks is customer support via e-mail. It's not something we screened for -- and not something easily noticeable when you don't interview candidates in English.

Are there any easy ways of quickly gauging a candidate's English skills, especially reading/writing? Something that doesn't take much time?