Asking the engineer to provide the calculations is not practicing engineering. Reviewing the calculations is not practicing engineering.
Questioning or challenging the calculations is not practicing engineering.
Explaining what, specifically, you are concerned about in the calculations is not practicing engineering.
Telling the engineer HOW to FIX the calculations would, in all liklihood, be practicing engineering.
The engineered beam for the 16' garage door should come with the engineers' calculations and loads used in design; the manufacturer employs engineers for that very purpose. I bought two when I built my house and my PE called the manufacturer, provided the loading, and was sent the calculations to justify the beam sizes. As a practical matter, had I not otherwise needed an engineer for the project, I could have requested the same thing from the manufacturer and submitted their design calculations with my permit application. Those two non-prescriptive elements of my house were 'engineered' and the 'engineering calculations' were provided.
Plan review exists, in part, to keep everyone honest in the process.
Good engineers don't mind plan reviews, most I know welcome them.
Those who can't be bothered to document their design, in my experience, typically have submitted a poor design.
Those who scream the loudest about plan reviewers 'practicing engineering without a license', in my experience, have just been caught submitting substandard plans.
We're all supposed to be on the same side; the owners. The Code Official has a duty to perform; the DP has a duty to perform. Both duties are owed to the building/properrty owner. It's all about making sure these buildings are safe.