How to check and ensure parallelism of front derailer and chainrings?
It depends on the derailleur.
Most or perhaps all Shimano FDs intend to have you use the top forward part of the outside cage as your reference surface. Where the logo is. Their published procedures show this. This is especially true of the knuckle type road models of the last few years.
SRAM Yaw FDs have you use the marked lines down the center of the cage in the back and front while looking down. Other SRAM ones use the same forward top surface.
Sometimes you don't know and have to intuit it. Usually you should avoid looking at the inside cage as your surface because it's typically stamped and manipulated all over the place. There are of course FDs, usually older, where the sides of the cage are parallel, but usually those are easy to align. (They're the sort most likely for you to need to angle the cage.)