Is there a way to know if someone has seen you're message

You get 15 minutes to edit a msg after which you can't. If your msg can't be edited before 15 minutes have passed then they saw your msg

No
Unless you can't edit it
And that's "your" not "you're"

The application of Black magic works.

Private messages
You cannot edit ones they've looked at

If you pay for VIP it tells you, otherwise no.

Only with a VIP account