premier/encore-how do I auto detect scene

I don't know of any automatic ways to do this; however, you should be able to do
this relatively quickly by hand.

Use keyboard shortcuts to move and insert chapters. Make sure you select your
video track first (click somewhere on it) and then use PageDown to move from one
clip to the next and * to insert chapters at the beginning of each new clip. If
you want to make transitions a bit smoother, Ctrl-D will insert the default
transition at that point as well. Even if you have a lot of clips, this should
only take minutes.

The process will only work if you have not yet merged all of your video into a
single stream. If you have, you'll need to hunt some more for a solution.

I have always found Automatic chapter points can end up messy.
Admittedly, I have never tried doing this in Encore - you can do this based on
intervals (insert asset to a timeline, and from the drop-down menu select the
"Add Chapter Points at Intervals" option.
I have used another authoring tool that allows automatic chapter creation based
on scene changes, but it's not reliable. You need to get the sensitivity just
right, but AFAIK it's not an option in Encore.
It would make a fine feature request though.