I have a project in which I have to know if a person is wearing a cap (the cap could be covered like helmet if required).
We started with using light sensors to check if the cap is occupied. However, it is very easy to bluff it by filling it with anything.
Why would anyone try to fool a cap?
And if there's a living person involved, wouldn't it be easy to check if the cap is running around without a person in it?
I think you have to reveal a little more detail, because it sounds a little absurd (no offense meant).
You can fool any cap (they're not very smart), but you can make it harder by combining a couple of detectors. Temperature have already been mentioned and anything outside a narrow band, say +/-5°C (with the center depending on ambient temperatures), would mean no human involved.
Adding a pressure switch in the sweatband would tell whether the cap is filled, but a small head may go undetected.
More info fom you may lead to more ideas!
So, we need a sensor that can detect if there is a human head inside or not.
That does not sound scary