For hardware types ask them to draw a full wave bridge rectifier.
At one place a recent-grad systems engineer was hired to do board repair. When asked to identify components, he said "I think those are resistors because they have colours on them."
He lasted about a week.
For a real engineer, ask how many soldering irons they own. We tend to upgrade from a simple one when we can afford it, but can't get rid of the old one because it may be useful for something. Then there are high power units for big jobs.
Have a small device on the table that isn't screwed together. Se how much time is spent looking inside, how long is spent operating it.
Engineers are more interested in what us inside something than what it does.
This assumes the device is a well-known gadget, not a functioning TriCorder!
What are the engineering and design challenges in creating successful IoT devices? These devices are usually small, resource-constrained electronics designed to sense, collect, send, and/or interpret data. Some of the devices need to be smart enough to act upon data in real time, 24/7. Specifically the guests will discuss sensors, security, and lessons from IoT deployments.