What are you seeing that makes you say this? The gray/discolored outter ring where the numbers are? Or the pitting/spots on the dial surface?
As someone vastly overpaid and under qualified once said: "the thing about science is it's not an exact science" (it's was a politician. A Conservative politician. OK, it was Ben Bradley and he deserves to be humiliated for that, and many other lines, for each and every one of his waking moments). The thing about science is it can be taught; it adjusts to new information; and, it can be replicated.
Working out how original a watch dial, and the rest of the watch, is actually is a science. There are a few basic rules: subdials should have clearly delineated engine turned markings; prints should be even and regular; they should follow the dial; and, hands should be pointing to the print. After those basic rules comes the experience part and that means handling lots of watches; reading old adverts; learning your way around various websites; and, trying to avoid boring and alienating your family and loved ones.
If you've got a subdial, look for those concentric lines. If you don't see them, then there's a layer of paint over them and, guess what .... redial.
Look at the print. That means the minute track the seconds marks the logo and any text. When it comes to the text you need a bit of experience because wonky letters and weird serifs can sometimes be correct but even without knowing exactly what a line of text should look like, you can easily spot what it shouldn't look like: letters that aren't aligned, serifs in different directions; broken or unfinished lines ... theyre all red flags. That said, there are much easier spots: where you have a subdial, the markers should reach the edge. That sort of applies to the main dial as well. A good rule of thumb is that the hands will rest on the markers. There are many exceptions to this and a lot of them come from the 70s when Omega went through its awkward stubby hand phase but for the classic watches, this is always a good indication.
This is often all you need to spot a reworked dial but the real geeks and nerds will be able to point out that the middle line in one particular E should be longer on that dial because it's the 1962 version which still had the coathanger S rather than the ones that came in with the later Singer dial, after the 4th of April that year, which was a Thursday and Antoinne had the tuna for lunch which always annoyed the rest of the the guys all afternoon but noone ever actually said it made his breath smell .... props to the folks who know all that but life is short.
Going back to your question, you asked if the fading or corrosion was a giveaway and it's not. That is condition, not originality. You can have a watch that's incredibly original but in poor condition; one that's in great condition but not very original; or, you can hit the grail of original and perfect. Frankly, the last scare me. I have some watches that are original and near perfect and live in constant fear of every door knob and shopping trolley when I'm wearing them. What this affects is value. That's all. The rest is personal taste. I have watches that fall in to all three of those categories. I sometimes buy watches just because they need saving and, sometimes, that really opens a world of pain. I have had to find original replacement dials on a couple of occasions. Now, I disclose things like that when I sell watches and it puts some buyers off. There are times that choice has to be made and I prefer a good looking, working, watch to something that isn't even a pretty paperweight. That also opens a different question: is a watch that is repaired with original old parts still original?
By now you must be thinking "I bet it's too hot and humid over there for him to sleep" and you could well be correct. This is something of a rambling answer because I'm trying to remember what I've written; simply alluded to; or, thought to put in and didn't. I can only see 8 short lines of text at a time and risk deleting the entire reply with one slip of the thumb. For those of you waiting for that, having read this far, sorry.
The final line of advice is this: handle as many watches as you possibly can. Come to each one with an open mind and work through it bit by bit asking the same questions over and over. Remember that you are only considering value and love each watch for what it is.
