The "email source" option (which is Cntl-u on Thunderbird) will show you the "Header" info of anything you send - after you send it, but it's better than nothing.
Here's the applicable header from your email:
Content-Transfer-Encoding: 7bit
... so you're not sending "Quoted-Printable" encoding. Most email clients have an option to send "Plain Text" (or something similar) emails. Some allow you to speify the option in the Address Book, and make it automatic. In Thunderbird's address book, the option is "Prefers To Receive Emails Formatted As ...", and you can choose "Plain Text."
Thanks for asking: I'll be happy to inform anyone who asks if thair email client is using Quoted-Printable encoding.
(Since this is of general interest, I've changed the Subject and reset the threading info.)
Bill