Everything you need to know about demodex mites is explained here, including how to do the test and find out if you have them.
The way I tested was to order some of their ZZ and put it on my face. When I broke out in whiteheads, I knew it was because the stuff was killing face mites.
You could also buy some cold cream at the pharmacy. I prefer Ponds because I like the way it smells, but Walgreens brand also works. Cold cream is a lot cheaper that zz. Put some on your face and see if your face breaks out. If you get a million whiteheads, you have mites on your face and the cold cream is suffocating them.
It took two weeks of applying ZZ twice a day for me to kill off the "mother load" of them. In other words I had thousands of whiteheads every single day all over my face for two weeks. One the majority of them had died, I still got 2-3 pimples daily for the next 6 months, after than I had 2-3 pimples a week for another two months. Then I switched to Cold Cream and I haven't had a pimple since. I applied ZZ twice daily for 8 months.
Cold cream kills mites on the surface, but zz gets to the ones that are embedded in deeper layers of your skin, where the cold cream can't reach. Once all those are gone, you can use cold cream for preventive maintenence to keep them from coming back. Put it on twice daily and use a hot wet washrag to take it off.
I lived with Acne for thirty five years, until I figure this out, and now I have had NO pimples. NONE in many many moths, almost a year. And I KNOW it is because I'm cleaning my face with cold cream.
I am not sure if it is possible to get rid of all of them with cold cream. If so, it would be a lot cheaper than using ZZ. You have to be willing to go through a couple of weeks of HORROR, I mean when those things start dying, you will have thousands of whiteheads. So pick a time when you don't have any big plans where people are going to be takign your picture.