How to assert that HTML is visible to user in email?

Question

I want some code or assertion to locate some static HTML code used in an email, then take in a user command in a short piece of text like "Transfer $5" or something. We can assert things like the number of divs that came before, or the number of nestings we are in, etc etc. What I want to be confident of, is that the text was shown to the user. There's lots of ways a higher level div can set body as invisible or take up 0 space, so I want to understand if there's a way we can constrain that some text was in fact visible to the user. It also can't be a brute force whole text match, since we'll be parsing this out of their email reply.

One idea might be an email attachment or an image since we can assert that the encoding is unchanged in the email and the image was attached. Not sure though, we want it to be lean and user friendly.