If you can locate some "reference" units -- Spice or earlier, or an early Voyager or Pioneer are (generally) examples with double-shot molded keys, and a later 32SII or most anything since as an example with painted keys, then looking at them side by side will help you appreciate the differences.
A careful look at most earlier examples with double-shot keys will reveal a very slight gap or fissure between the contrasting colors -- presumably the legends were injected after the keys themselves had hardened, and along some edges, the plastics don't meet exactly flush.
Later, as in middle-era Pioneers and HP-48's, it appears to me that H-P went to "grinding" down the keytops after moulding to remove the upper, flawed layer and reveal deeper slice of the keytops where the plastics were more uniformly filled in. The upper surfaces of these keys tend to have a dull, "sandpapered" look, that constrasts with their shinier sides. The top surfaces sport fine horizontal etching or grooving, suggestive (again) of sandpapering or grinding.
Either way, the double-shot legends tend to be larger, more fully formed, and offer indelible contrasting color that is unmatched by painted-on symbols. Again, a side-by-side comparison will reveal noticeable differences. (The differences to be noted between early and late examples of the HP-32SII are so striking that they'll bring a tear to the eye . . . )
The painted (or silk-screened?) legends on SHARP calculators, and lately the TI's and HP's 49G+ (among, I'm sure, many others) exhibit noticeably raised lettering when viewed at an angle in the glare of a bright point-source light. (Somebody help me with the technical jargon here . . . ) These clearly have a thick ink or paint laid on top of hard, plastic or metal keys. IMHO, SHARP has always done a good job of this, and the new HP-49G+ keytops are very crisply printed and offer great contrast, though some alignment problems have been noted in the printing on early examples of that model.
Some early models' painted keys have the paint laid into moulded-in depressions for the legends. These display obvously exaggerated relief, and tend to collect grime that softens the legends' contrast.
I don't think rubber keys work in any way, and their painted-on symbols tend to wear off the soonest. Of course, in general, any loss of the keytop legend indicates a painted key.
I think an article with close-ups would be a worthy contribution to The Museum -- I'll see if I can get 'round to it.