The ones that check your cholesterol are not to be recommended. For a proper cholesterol test and, indeed, many other tests, you are supposed to fast for 12 hours beforehand for an accurate reading.
A few years ago I went for a bone density test, and, since then, went for a proper one in the Department of Anatomy in Trinity College. I was told there that the test I had undergone was inherently flawed - I had the printout that I had been given.
Basically, these roadside checks are money makers.