I'd like to remind the lottery rules:
Check off the Encrypt Message box, the message must be publicly visible on the blockchain for transparency.
Encrypted attachments will not be processed, funds sends with encrypted attachments will be considered donations. Some people send encrypted from time to time. Don't unless you want to make a donation.
How can one tell (after the fact) whether or not they sent an unencrypted message?
In the NXT client when you click on send NXT - Add a Message, there is a Encrypt Message checkbox. It's checked on by default, you must check it
off. Checking off will send the attachment unencrypted.
When you click on a confirmed transaction in the NXT client or
https://nxtportal.org, you can receive information whether the attachment is encrypted or it is not. In the latter case you will be able to read the contents, and everyone can. The lottery can't process encrypted attachments because then there is no way to prove who sent what numbers. The numbers must be public and visible to all on the blockchain.