Easiest way I find to determine if you'll be on a PMCO aircraft retrofitted with E+ is just look at the seat map of the route you're considering; of course there can always be post booking aircraft changes as well though.

I had such a change work in my favor though for an upcoming flight. I was on a non-retrofitted 753 and booked my seat near the front of the aircraft. Now being a lowly silver I would not normally be eligible for E+ unitl t-24, but then they made an aircraft switch to a retrofitted 753 and low and behold I kept my same seat number an am now in E+.