Here are some thoughts on foxes from Isonokami Nobutsugu 石上宣続's Bōka enman roku 卯花園漫録 ("Deutzia Garden Miscellany"), a collection of trivia apparently compiled in the late 19th to early 18th Century.

It is said that foxes are yin by nature, so that even most male foxes will transform into a woman and have relations with men.

I never thought about this before, but it's true. (For values of truth encompassing fiction. You know what I mean.) On the other hand, I'm pretty sure Bugs Bunny dresses as a woman due to an excess of yang, so I'm not sure how much that has to do with it.

When one cannot tell whether another's suffering is from illness or the work of a fox, brew the leaves of the Japanese star anise (shikimi) and have them drink it. If possessed by a fox, they will refuse out of fear. If truly ill, they will drink it though they complain about the smell.

It turns out that shikimi is toxic and shouldn't be consumed by anyone. I suppose that the caution against seeing foxhood in mere reluctance to drink a cup of liver-failure tea was added to prevent false positives arising from this fact. ("Hey, this guy doesn't want to drink poison!" "I knew he was possessed by a fox!")

Incidentally, my source for Isonokami is Yoshikawa Kōbunkan's Nihon zuihitsu taisei edition (volume 23), ed. Nihon Zuihitsu Taisei Editorial Dept. (日本随筆大成編輯部).