How Do You Know If It's Been Dipped?

If it had only one or two quick dips and thorough rinses, no one could tell. Repeated dips and insufficient rinses/handling are what dulls the luster and provides evidence of dipping.

Peace dollars don't easily tone (wild toners are the work of coin docs). Morgans, on the other hand will tone easily if stored in an environment that is conducive to toning (craft envelopes, certain coin folders/albums, paper coin rolls, etc.). If the coin still exhibits brilliant luster, who cares if a previous owner properly removed unattractive toning. Just make sure there's no remaining residue on the surface of the coin.

Why don't Peace dollars tone as easy as Morgans? Aren't they both made of the same metal content?
John1

As to the original topic you should really explain if your saying dipping or cleaning. Could be a world of difference. Many dip coins in solutions like Acetone and only briefly to attempt to get rid of some minor contaminates. Such dips usually don't take off toning, corrosions, tarnishing, etc. They do however, usually, not always, remove tape glues, glue, melted plastics, etc. Such minor, short timed dippings are really almost improbable to notice.
The numerous variations in what may be termed cleaning is very much something elses. So called dipping in solutions such as Jewlery cleaners, acids, different juices such as Lemon Juice constatute cleaning and if done can almost always be noticed. But then too, if done properly, some of those also are difficult to tell.
If many say a coin has been cleaned, it usually has been, but not necessarily. If many say a coin has not been cleaned, it only means they could not tell for sure, so it could have been cleaned or dipped but since it does not show, then ..............