Yes you won't know anything. Using that service is just copying the mesaages to be saved via email or another number. All u will know is that they got it or read it. Nothing else. Unfortunately there is no way to know that information unless the person u text tells u they did it.
You won't know that they are using this service at all. Meaning u won't know they are sending the text messages between u two to a email. You won't know anything UNLESS the person u are texting tells u. The option is only set to be used to the person that turns it on. As a privacy the other person won't know anything. Some devices have a option in the settings that can turn on a Read or Delivered status but it's mostly for MMS and not text messages