We had an 8:15 ADR for CRT and they let the early ADR line into MK at around 7:40. It didn't matter if your ADR was at 8, 8:15 or 8:30, everyone was in the same line.
That is what I would bring, or print it off Disney's site. Better safe then sorry.
When my wife and I went this past May we had a breakfast at Crystal Palace for 8:20. Stood in the line with other people waiting to get in also and at about 8ish they let everyone in. Never stopped anyone asking for a confirmation number, nothing. While we were getting a picture taken in the Hub a group of teens (15-20) went and stood by the ropes to head over to Splash Mountain. Which was bad for them seeing it was down, until about 11:30 that morning.
We also had another breakfast ressie at CP our second to last day. While putting our tickets in the ticket taker the guy asked us where we were eating. Never asked to see anything, but we didn't get into the park until about 8:10 this time and the line of people were gone by then.