OK then if you want to know more.
I have interviewed people, a couple of times when I have left and found my own replacement.
Because your starting out your experience is obviously very little, so your actual level of understanding isnt expected to be that of someone twice your age.
Employers ought to be looking for lads that are enthusiatic and actually want to learn the trade, I said 'ought' because in my experience personnel departments these days are impressed by silly buzzwords and people researching the business they are applying to.
As far as technical knowledge, the interviews I've attended (interviewee) there has usually been a set of drawings and questions have been asked to get me to show that I understand the drawings and various parts of the machinery, pumps, bearings, gearboxes, drives, contactors etc, also a few times theres been a box of parts on the table and I've been asked to identify them (if your canny you can tell a lot from how someone handles something, an experinced engineer will look for rating plates/signs of damage etc autonomously), inductors, proxys, clutches, capacitors, braking resistors, bearings, plc i/o cards, it might be worth while studying some machine drawings and looking through an engineers merchants catalogue to familiarise yourself with the general idea.
I'd be surpised if your asked any formulas, if you are then maybe ohms law, 3 phase power equations, zigler nichols pid, stored energy in a flywheel etc.
To prove your usefullness with scada you might also be expected to demonstrate some knowledge of pc hardware, including some dinosaur cruddy stuff, a lot of scada I've come accross dates to pre windows and uses at slots instead of pci.