How can someone tell if a stamp is a re-engraved issue?
How does re-engraving affect the value/collectability of such an issue when compared with the original(?) version?
Most often, catalogs will detail what a re-engraved issue will look like. With no other reference or a-priori knowledge, one wouldn't be able to tell if a given stamp is re-engraved or not. There are usually small differences between the original and re-engraved issues that often can only be seen under magnification (sometimes not though).
Value/collectability is determined by the market and usually, but not always, based on catalog values.
Robert, can you specify which country issues you are considering? (Or if it's purely a world-wide question, then Greg's generic answer will do).
Robert, I hope I can help but this is difficult to answer concisely. Re-engraving is not a "condition," like mint or very fine. It is a modification of an original design. There are a number of reasons why a stamp design might be re-engraved. Perhaps to repair damage, to differentiate a second or third printing, to correct a mistake, to change the face value or purpose, sometimes to simply clean up a muddy first effort. The major catalogs will describe the changes and re-engraved designs will most often be given a separate listing. Value-wise there is no relationship between the first effort and any reworks of any design. Essentially, they are different stamps and whatever their values, they are equally collectible.
Phil & Greg - thank you so much!
Scott - mainly British Empire/Commonwealth. Many U.S.
So for US, there are 4 issues that have special merit. These are the 206, 207, 208 and 209. In this case they were released as an acknowledgement of the merger of two of the banknote companies. They are easily distinguished (aside from the 207 which you have to work at a little bit harder to identify) from their heavy lines, and in the 208, it's shade is significantly different from the other banknote issues.
Don't confuse "Reengraved" with "Recut" as well. Some plates in early stamp printing would become worn, and the details obscured of the design, these lines would be "recut" and afterward will appear really heavy compared to the other lines around it. (Very common in the 10, 11, 25, 26, 10A, 11A (especially here), 25A and 26A). Also the 1851 and 1857 prints of the 1c Franklin (probably the most complex of all the issues to distinguish due to their close proximity of design to very thin boarders, and details are often cut off making it difficult to ID without plating).