Chicky: This is how I would approach the problem:
Check out the Wikipedia page on the Walther PP. It lists the variations of these guns, which is a bit convoluted. From this point, you can start searching the web to perhaps get an idea of how old this gun is.
Another trick might be to check out this site, where bluing is described:
https://kurtthegunsmith.com/gun-bluing- ... miths-use/ If this one doesn't help, google types or kinds of bluing. The idea here is to note the different types of bluing and the colors they produce, and then find out what kind of bluing the Walthers of your gun's age used. I picked this site because the descriptions are short, but you might be more comfortable with longer descriptions or better pictures of examples. For instance, the caustic bluing that's often used today may be durable, but the "Colt Royal Blue" is instantly identifiable compared to the black caustic blue on modern Smith & Wessons and Rugers. Going over Walther pictures on web sites might help, but you know that colors are often not true when portrayed on line or printed, so here you don't have a sure thing, just an indication. The idea here is to match the bluing on the gun with what was originally used.
Check the clues:
How old is the gun? (This goes back to the first point about identifying what you have.) If a gun is, say, 50 years old, did it come in a box? If it looks like new, but the checkering on the grips is worn, this is a clue. Of course, the grips might have been replaced, but if one portion of the gun doesn't match another in wear, you probably have come across a refurbishment.
Edges and corners: Reblued guns are often polished to some degree, and there is certainly some workmanship involved in keeping edges sharp. You will find these on the machined edges and ridges of the slide, and on the serrations used to grip the slide. Are they still sharp, or do these seem dulled or "soft"? What about the serrations on the hammer spur? I don't know if these guns had a serrated trigger, but some guns do, so if these are present, are they worn smooth or still sharp? This is like when you look at a used car and the paint is all bright and shiny, but you notice that the rubber brake or clutch pedal has wear on it that doesn't match the new look of the car. The edges of the sights will wear if the gun has been placed in a holster and removed a number of times.
Flaws under the blue: I have a 91/30 Mosin Nagant made in 1938. It looks very much to be brand new. Of course, considering that the Soviet Union fought a terrible war after the gun was made would tell one that a brand new gun is not likely to be found of that vintage! So, looking at it, even though the blue is new, there are some pits of rust at the end of the barrel: a sure sign they were too deep to polish out and were blued over. This would also apply to dents, scratches, or gouges that were left after bluing.
You can also pull the slide off and check around the feed ramp and even the feed lips of the magazine. Check where the magazine is inserted into the grip.
A lot of this won't help if the gun was only very lightly used, but then reblued.
Rust is a chemical reaction involving iron (or steel) and oxygen, and we know that many reactions are enabled or exacerbated by heat or the presence of moisture -- two things that aren't rare in a lot of India. If the gun has rusted at some time, it will have to have been polished some before bluing. Also, rust can take the form of little pin sized pits that aren't evident until you get the gun up close and really examine it. Use a magnifying glass, if you have to! I carry one around all of the time, now.
Of course, when you have the slide off, check the barrel for wear. I would think that you shouldn't see evidence of barrel wear at all, or maybe just a slight amount.
You would also want to check the operation of things, such as how the trigger feels when pulled, and whether the decocker works correctly. Incidentally, what about the decocker lever? Is there a wear pattern it forms on the frame when operated? This might give a clue.
Another place for wear that I can think of is, when you pull the trigger guard down to remove the slide, see what the edges of the catch mechanism look like. Are they worn, or still sharp, like you'd see from the factory?
These are some ideas that I would look for. It's like being a detective, where you look for clues and try to come up with the best deduction of the situation from analyzing them.
This is about what I can come up with as far as things to look for. Good luck!