You are looking at a coin which on first view appears to be a used or circulated coin (tarnished, maybe a little wear, maybe a few minor scratches/dents). But objects placed close to the coin reflect fairly clearly in the coins' field, which suggests the coin may be a proof. First strikes (particularly) of circulation and uncirculated coins can also have mirror like finishes similar to proof coins, I believe. It is my understanding that most proofs usually have frosted details with clear mirror like fields, although reverse proofs have frosted fields and mirror details. Uncirculated coins (coins intended for collectors/display) usually have mirror like finishes for all of the coin, although some may have frosted details with mirror like fields. First strike circulation (or business) coins usually have a mirror like finish for all of the coin. So, by examining the coin ONLY (no referring to books), how can you tell for certain whether the coin is a proof, uncirculated, or circulated coin?
Some of your thinking is a little off. Business strikes, coins struck for use in circulation, do not have mirror fields. Yes, there was a time when some did, but that was long ago, over 100 years, and only very few were what are called DMPL (Deep Mirror Proof Like). Your questions are lot more complicated than you might think. But that is just because minting techniques have changed so much over the years. But in simple terms there are only 2 methods of manufacture for coins - Proof and business strikes. Proofs and only Proofs will have typically have the mirrored fields and frosty devices. Business strikes will not have them. Proofs will have much better detail because they are struck at greater pressures and they are always struck at least twice. Business strikes are struck at lower pressures and struck only once. Proofs, unless they have entered cicruclation, will rarely have marks or dings all over them. Business strikes almost always have marks and dings all over them. Proofs will have square rims with sharp edges. Business strikes will have rounded rims with soft/eased edges. Proofs will exhibit a luster that is highly reflective and that cartwheels in a thin line of reflected light when turned under a light source. Business strikes will have a non-reflective surface and the lustrer will cartwheel in a thicker, wider line of reflected light, much like the spokes of a wagon wheel spinning very fast, when turned under a light source. Those are the general ways a Proof can be distinguished from a business strike. Now if you wish to get more specific, then you will have to pin down the time periods you wish to discuss.
Thanks for the helpful detailed reply! Currently, I am looking at two of my coins and wondering how it can be certain whether they are proofs or not. That was the reason for my question. I have a New Zealand 50 cent coin dated 2006. It is supposed to be a circulation coin (I think). There is some evidence of very minor scratches/bag marks on the fields. The coin has a mirror like finish; adjacent objects are clearly seen! Another coin I have is a Great Britain farthing dated 1930. This coin is supposed to be a proof (according to a dealer). It appears the coin may have been handled, as there are some possible partial fingerprints on it(?). There are also very very slight dents/scratch marks on the fields. Very slight darkish brown tarnishing is apparent. This coin has a mirror like finish also, but perhaps not as clear as the New Zealand 50 cent coin (possibly due to the tarnishing?). I wish I could upload photos of the coins, but it is proving difficult to obtain decent photos (the camera doesn't see what the eye sees)!
Tell ya what. Go to this link - http://worldcoingallery.com/ - find your coins, post links to them here, and then we'll talk some more.
There are foreign uncirculated coins with proof like appearance, what GDJMSP mentioned apply to american coins, some foreign coins are processed differently, so what you said might be correct. http://stamps.nzpost.co.nz/cultures/en-nz/coins/
