I used the Billings Method for over 12 months prior to this PG (it was a planned one BTW )
After a few months I was able to notice the slippery sensation. It was a great topic of conversation "not tonight dear I've got the slippery stuff" lol.
Is the online instructor a new thing? At the time I was teaching myself this wasn't available. I called the help line in the book and was contacted by Evelyn Billings and had a great chat to her about all things CM. She told me you still need to be aware of the stretchy stuff but to be wary that you can be still quite fertile once this has passed and you are still having the slippery wet sensation. This is more important if you are using it for a contraceptive reason in which case you don't BD for 3 days once you have noticed the CM change back to creamy or dissapear and feel dry.