I've never heard that there is anyway of knowing if your are just typing it in. I know that it was very common for us when we got to some candy item that was on the checklanes our grocery TL had a tendency to type in that DPCI and just say that they were "OK"... All I know is that we would get yelled at if it wasn't completed every day. Just another one of those duties that I had to take care of a few days a week that I didn't have time for!
Yes, they will know. However, I think there is less of a chance of the ETL caring than there is that the DTL will care. Its OK if there are a couple a week, but don't make it a habit. There are very few situations in which it is appropriate to not scan it.