EDIT
The original answer I posted here was incorrect. I wrote that "client.ssl.enabled" should give you what you want, and further that this variable isn't documented clearly. I now think both of these statements are wrong.
You can check "client.cn" - if it is null then the client has not authenticated. If it is non-null then the client has authenticated.
INCORRECT:
I think "client.ssl.enabled" gives you what you want.
LinkedIn
Twitter
