Most, if not all of the Delta gates at LAX now have the "boarding zone pylons", I believe. There should be a several posts in the boarding area (sometimes they are mobile and the gate agent doesn't put them in place till just before boarding begins). Each post has a boarding zone on it, which corresponds to the zone on your boarding pass.

In my somewhat limited experience, when I have arrived later in the boarding process and stepped up to the Premium pylon, the GA called me to the front and scanned me in next. I can't say that it was always worth it though, because after that I was in the same line as everyone else, creeping down the jet bridge to get on the plane. based on those experiences, if I can't board with the Prem zone, I will usually just wait till the end - since it doesn't take any more time than standing in line.