Neither can they. No they can't and neither can the grading services. That is why PCGS now calls ALL 1856 FE cents proofs, because they can't consistently tell the proofs for the Unc's and they were being called on it too much. And both services have a lot of difficulty with the copper nickel proofs of the 1880's and will often call pieces that may be Uncs proof just to cover themselves (The business strikes are much more expensive and they don't want to be on the hook for the high cost of having to pay out on the guarantee if they are wrong.) And there are other series where it can be very difficult to tell the proofs from the business strike coins as well. Back in the early 2000's PCGS ran ads looking for graders and offereing salaries of "up to $250K". Of course most would start out at less. But even a $100K salary may not be that great when you consider that PCGS is located in one of the most expensive places to live in the country.
Conder, that's right, and I'll take my licks for over-generalizing. But they can tell as well as anybody. When I say a kid who's serious about the hobby I'm saying serious enough to acquire the requisite knowledge to appreciate what they're looking at in a coin, because that's what they're in the hobby for, to appreciate coins. That issue of MS or PR comes up in these forums all the time, and sometimes we're even stumped. The fact that there are aberrations that stump us doesn't betray our analytical knowledge, if that should make any sense. This hobby on the whole lost that appreciation of coins once it got off technical grading and into the irrationality of "grading" coins for the market, as so many commodities, but then that's perhaps an issue for some other thread.
I think everybody here is well aware of my opinions regarding the TPGs. But I'll say this, their graders know more about coins than 99% of the people involved in the hobby. Now you can believe that, or not believe it. Doesn't really matter.