What objectively separates engineering questions from experimental physics ones?
I will argue that trying to formulate such a distinction is not the correct way to attack the problem of deciding what questions should and shouldn't be considered on-topic for this site.
A better approach is to ask whether or not physics or engineering flavor is more appropriate in the answer.
It's a question of answers
Suppose I want to build a waveguide resonator with low dielectric loss.
I could post the following question to either Physics SE or Electrical Engineering SE:
How can I design the geometry of a waveguide resonator so that it has the minimal dielectric loss?
If I pose this question to an electrical engineer, I will likely get design rules, possibly in the form of immediately useful formulae.
If I pose it to a physicist, I may learn something about the physics of electromagnetic fields near dielectric boundaries, diverging fields at conductor corners, and maybe even something about conformal mapping.
Heck, if I pose it to the right person I might learn about numerical recipes to solve electrostatics problems.
This example illustrates that the appropriate place for a question can depend on what kind of answer the person posing question wants.
It would be wise to keep this in mind when judging the appropriateness of questions on this site.
Before knee-jerk voting to close a question, we must ask ourselves if the asker may have had a reason to post it here instead of somewhere else.
Who is reading your question?
In our capacity as experimental physicists, my colleagues and I spend >99% of our time building equipment, fixing equipment, and programming a computer.
These are all activities which could be classified as "engineering", yet we identify as physicists.
I frequent Physics SE and not Electrical Engineering SE.
This is not without reason: I am far more qualified to answer questions here than I am on Electrical Engineering and, even when I'm asking an engineeringy question, I would rather get the benefit of people who work in similar environments and with similar equipment as myself.
The point here is that another important aspect to deciding whether a question should be categorized as "engineering" is the readership of the site.
If I ask a question about a machine, the Physics Lab Quizzwopper 2000X, nobody on Electrical Engineering SE will ever have heard of it.
So, while a theorist might find a question about equipment off-topic, an experimentalist will not, because the experimentalist wants the answer which only a physicists can give.
Audiences aren't well delineated
Consider the following question:
How can I use the symmetries of physical system XYZ in order to vectorize my simulation program in computer memory for optimal performance?
There is no way to objectively categorize this question as "physics" or "engineering".
All we can say is that the person most like to give a useful answer to this question is someone who has solved a similar problem in the past.
This could be a "physicist" who had to learn some programming or an "engineer" who had to solve a physics problem.
In this case, it's probably reasonable to let the question sit wherever the asker put it, and if it doesn't get any attention, that person can move it somewhere else and try again.
P.S. I have avoided proposing an objective criterion by which to decide whether a given question is appropriate for this site, as that issue is not strictly within the scope of the original question.