There is a printed catalog of Bernard Goldberg cachets by Gerald Leviss.
Posthumous hand-painted cachets have been going on for a long time. It has been done for FDCs, US naval covers, US pictorial cancel covers, you name it. "Posthumous" can mean a day or decades. The famous FDC artist Dorothy Knapp started producing cachets for friends and family way back c.1940 but also added cachets on older FDCs as well.
Unique pen-and-ink drawings/cartoons on covers go farther back to the 1800s. But these have also been added/faked in the 20th and 21st Centuries.
Sorry, but I dislike the cachets on exposition covers not only because they are posthumous by about a hundred years, but because they are in the style of Goldberg, fine if you are collector of his work. They are not at all in the style of the period. Your mileage may vary.
Hand-painted does not necessarily mean unique. There are ones with an edition of 50 or more with basic outlines printed and colors added in watercolors, sometimes so hastily that collectors would only want the best ones. Hand-painteds will range from using templates to do the basic outline work to largely unique designs for the same subject.
How to tell period items from modern posthumous creations? You can look up a modern artist and find their work easily enough. Otherwise, you'll have to learn the styles of the period and paints and inks used, so that means you often need the cover in hand rather than looking at a scan online. There are many unique creations by unknown artists so that that skill is useful to identify period artwork.
Here's a test. Is this an add-on from years later?
This is a naval cover for the recommisioning of the submarine S-11 (1940).
Somebody did the favor of writing "Fleckner" in pencil at the bottom, so it was relatively easy to find. The cachet (that does not match the subject) was done by Mae Weigand, who among other things produced two very large series of hand-painted patriotic flag cachets on various covers including a few naval subjects (like this). Posthumous in the sense of not being sent with a pre-existing cachet, but certainly produced right around the same time.
I think I can see a slight trace, but event covers at this time were supposed to be addressed. There are Weigand covers showing light pencil addressing so it was probably erased here.
The black ink and paint type is typical for the time. The black here is drawn and not printed since there are variations from cover to cover. The cover follows the hand-painted "rule" even today of not painting over the cancel with a solid color. Pale/dilute colors do not interfere with cancels.