I assume you want to know how to tell the difference when more than one method is used to print a specific stamp (e.g., Machins).
I don't have access to my scanner right now, so I'll do a rough description. I'm addressing the specific 3 methods you mentioned, and not the other variations (e.g., gravure, photolithography, typography...)
On most (not all stamps) printed by more than one method:
photogravure -- everything is converted to a dot matrix screens or half-tones, so if you look very closely (easier with a magnifier or high-resolution scan), everything is composed of small dots (even what appears to be solid colors); ultrathin straight lines/edges will sometimes appear to be jagged under very high magnification, depending on orientation
lithograph -- oil-based printing process, so solid colors will be solid; lines will be straight, but line edges can be uneven and not as sharp compared to engraved printings
engraved -- basically, what you etch is what you get; background often composed of lines and dashes, solids will usually be solid, but much more extensive use of lines and dashes for background shading; lines/edges will be very sharp
If someone doesn't already do so, I'll try to scan and upload some pics next week.
On some stamps that have few or no thin lines of shading, it can sometimes be very very hard to tell between engraved and lithographed printings. On engraved stamps, the ink will largely be sitting on top of the stamp surface (i.e., the colored design is "raised"). So if you have sensitive fingers and run your fingernails gently over the surface of the stamp, you can "feel" the design. As a LAST resort, you can always try the aluminum foil test -- put a small smooth piece of thin foil over the surface of the stamp and gently rub over the foil with your fingernails. If it is engraved, you will start seeing bits and pieces of the design appear slightly embossed out of the foil surface.
By far, most stamps printed today are through lithography or photogravure process. Engraved stamps are the exception because of the time and expense. But a lot of traditional collectors like the engraved stamps.
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