1) how does the optician know which is which - they either check the prescription and then check the properties of the lense by observation or else they look for the little R - but this is easy if you are an optician and have the relevant bright lights and magnifying lenses. If the presecription for my right eye says (for example) +2.5 and the optician checks the lense and sees it is +2.25 then they know that the wrong lense is in the wrong eye.
2) using glasses to check - I have tried to do this, but still cannot see any significant mark which would give anything away.
3) henny - I can't bring myself to justify throwing away a perfectly good pair of lenses every night. I know they are designed for single use, but to me it is like eating from paper plates evey night and just throwing away the dishes - wasteful. I also object to paying opticians for recurring supplies of the lenses when they don't actually need to be thrown away.