Proof & DMPL
First, we will discuss proof and deep mirror proof like coins, commonly referred to as PR and DMPL respectively. A proof Morgan Dollar will not always display a noticeable contrast between the bust and the fields. Although many will, some will attain a satin appearance or toning that takes away from the Deep Mirror appearance and contrasting features. Proof coins just tone differently because they are struck from specially prepared dies called Proof Dies.
These dies are prepared differently then business strike dies, and often are superior in quality then business strike dies. However, there are many PR Morgan's that have weekly struck features. Proof Coins struck from proof dies must never be confused with coins struck from overly basined business dies, that causes a struck business strike Morgan to display a DMPL appearance. A Deep Mirror Proof Like coin is not a proof coin, and is no way related when it comes to the dies they where struck with.
Morgan Dollar Working Dies where basined before striking planchets, and in this process the working die was placed in a zinc receptacle full of fine grit. The longer the die was in this receptacle the more mirror finished it became. This resulted in many Morgan Dollars obtaining a DMPL or PL appearance.
Also, a Deep Mirrored coin will show raised "striations" in the fields from when the metal was stretched or thinned to obtain the proper thickness. These striations must not impede the reflectivity in the fields for a Morgan to be DMPL, but most where polished off during the basining process for the mirror fields to be deepest. Below is an example of a true Deep Mirror Proof Like Morgan Dollar.
In other words, a Proof Morgan Dollar was struck with a specially prepared proof die, and a DMPL coin was not. With experience the collector will be able to distinguish the different characteristic between the die strikes so as not to confuse a proof with a DMPL coin. However, PR Morgan Dollars are rare, so the main point is distinguishing between coins with normal and striated fields and deeply reflected mirrored fields.
The best method is a white piece of paper and a small ruler. Place the ruler on a white piece of paper, use cotton gloves to hold the coin at a slight angle, and if you can clearly see at least the 6 inch mark clearly on the ruler, without distortion, then it's a DMPL coin. This does not apply to coins that have been polished by a dealer or ebay seller. Viewing the coin under a 10x loupe or microscope will determine if a coin has been doctored, but it might not be readily apparent to the inexperienced eye, so a submission to a reputable grading service might be in order.
(Note: Not all Morgan Dollar dates where struck as proofs. No proofs where struck in 1921 except CHAPMAN & ZERBE proofs and most of these don't have PL surfaces and are so rare that to buy a supposed ungraded example would be foolish. Much debate still remains among experts concerning the Chapman and Zerbe proofs).
Also, it should be noted, that many proof coins only obtain a PR label and are not always cameo or deep cameo. As stated above, a proof is struck from special dies and this is an obvious diagnostic for the trained eye, but's not always evident in photographs. So don't be fooled by seller's claim that a coin is related to a proof just because it has proof like or a cameo appearance.
Below are three examples of proof Morgan Dollars for the viewer's reference. As you can see the appearance of a true Proof Morgan has a mellow luster and often deeply struck details. Most Morgan Proof mintages are under a thousand coins, or just a little over a thousand, so they are very rare and are not usually found on ebay. If you do find a proof up for bid on ebay, I suggest not bidding on the coin unless it's attributed by PCGS, NGC, ICG or ANCS.