How To Tell If A Coin Is Cleaned?

You may get a lot of answers to that one. However, I will throw in my 2 Cents here. First of all if you see a coin that is well or even slightly worn and has complete original luster, I would suspect it has been cleaned. If you find an extreamely old coin and still looks brand new but has some dirt inside the letters and numbers, it may have been cleaned. What you have asked is in fact a really tuff question. I've known dealers that have sent in cleaned coins to a TPG service and some have gone through with no mention of cleaning and he admitted he cleaned them. I too in the past have done that. In other words even the so called top of the line grading services can be fooled. Some coins are very apparent that they have been cleaned and others just can not. People that think they are experts in this field have seen many coins they would accept as not cleaned when in fact they were. The main problem is if a coin is cleaned really carefully and has little to no wear it is difficult to say cleaned or not.
My best example of that is a 1995 Lincoln Cent Double Die. I purchased it at a coin show for an astronomically cheap price due to it had fingerprints on the obverse. This coin was in the MS-64 or higher grade. With nothing to loose I cleaned it with Tarn-X, Acetone, Alcohol and finally some off brand stuff from Walmart.
When I finished the fingerprint was gone. I took that same coin to several dealers I know at a coin show. NONE thought the coin had been cleaned and all said great find.
Unfortunately that is the trouble with a cleaned coin. On some you just can't tell. I would suggest you try cleaning some very common, old dirty coins to see what they look like.
Here is a site that tells you many ways to clean a coin.
www.pennycollectors.com

yes, the cleaning reduces the value.
Easy way to tell: if it looks to nice, it was probably cleaned.
(Just kidding) but, nearly every early coin was cleaned at one time.

Becky, I agree with justcarl. A really good way to tell if some coins have been cleaned is to clean several common coins yourself using various methods, such as ketchup (for cleaning copper) alcohol, tarnex, acetone and so on. Keep those and how they were cleaned. Use them if you run into a questionable cleaned coin. Hands on experience is the best.

Becky. One more thing. Occationally you'll see or hear of someone sending in a coin to a TPG for cleaning. That one has always puzzeled me. If you clean it and send it to them, they'll usually send it back in a bag saying cleaned. But if you send it to them, they clean it, place it in a slab and all is OK. I just don't understand that one.
Primarily any coin or old item being cleaned will reduce it's value. If you ever watched that program on TV about the Antique Road show they always emphsize not to clean, fix, etc any old item. You'll hear them say if this was not cleaned it would be worth XXX more than now since you cleaned it.
In the past people did just the oposite. In the old days it was common practice to clean old coins, shine them, even do anything to make them shine more. Now we realize in most instances much of the original metal has been removed in the process.
Yes, cleaning a coin reduces the value.