You don't need test equipment to see if someone is wearing a helmet, but you are right on reflection, perhaps enforce is the wrong word perhaps police would be better.
I'll just nail my flag to the wall where I stand on the whole issue of helmets then retire from the debate. I wear one because it gives my family piece of mind and that's the only reason. I am against compulsion as when it is found that helmet use will not significantly decrease accidents (if at all). compulsory training will be next on the agenda, then of course they will need testing to make sure the training is up to standard, then of course you will need licences to prove you have passed your test. That unfortunately imo is what will happen.
I have only my ignorance to guide me, but I'm not too sure about compulsion being a real threat.
The general public (the electorate) appear to find current accident, injury and death rates among cyclists acceptable.
Unlike the motor industry, who worked hard to support the mandatory fitting of airbags in new cars, the cycle-safety lobby does not have a bottomless slush fund. We are approaching the stage where cars will be cheaper to scrap than to pay for re-charged airbags. The obsolecence curve in new cars gets ever steeper as new, more rigorous safety laws are passed.
Even the scrappage scheme (politically brilliant) was really a handy transfer of money to the motor industry from the exchequer. Brilliant, but a product entirely of creatives in the SMMT.
The road-safety lobby do not have that sort of money or muscle. The SMMT can do a lot better when it wants Bills read than to give them to a low-profile Lib Dem MP from Devon....
I'd rather the cyclist accident, injury and death rates were lower, but I'm not in favour of helmet legislation or training legislation being passed in an attempt to alter them. Nor is anyone with whom I've ever discussed the matter is in favour of either measure. That's not a hugely credible statistic, but it works for me up to a point.
I agree with rowan46 that one outcome of compulsory training would be a large, bureaucratic poo-shaped object, but (basing my opinion largely on ignorance) that is why I do not think it will happen.
Dare we dream in our worst nightmare of a sister branch of the DVLA, the CBLA? I don't think a reading in the H of C would survive the first mention of cost.
In a way, fear has worked with the wearing of helmets. When I started to ride, almost nobody wore one. These days almost nobody doesn't.... I still often go without, but when I do wear one it is because of superstition and an irrational fear of something that hasn't happened to me in scores of high-speed offs....