Upon discovery of the error the USPS recalled all the stamps that had been distributed to post offices and printed new stamps with the correct picture.
Everything might have been okay until it was discovered that a few sheets of the recalled stamps had been inadvertently sold before the recall, and with the recall, those few sheets would have been instant rarities, potentially worth thousands of dollars.
Collectors demanded that the recalled sheets be issued anyway, but the lucky few who already had the sheets sued the USPS to prevent them from releasing more to the public. The lawsuits failed and the USPS eventually released 150,000 sheets through a lottery.
If your sheet is in a blue card stock envelope with celaphane windows you can be reasonably certain you have the rare sheet, and if it's not in the blue envelope it's probably the common sheet, unless you're one of the seven people who bought a sheet before the recall. (My guess is that you would know if you were one of those people.)
Even so, it's always possible that your sheet was removed from the blue envelope, so then how do you tell?
Compare your sheet to the two pictures above. The picture on the left is the recalled, and rarer of the two. Notice that Ben's right sleeve touches the red frame that borders the stamp. Also compare the hats and faces, in the recalled stamp Ben looks much younger and his hat is in better condition. For the pedantic, notice that the red frame that borders each stamp is dark blood red on the recalled sheets, and is bright red on common sheet. You'd probably need one of each sheet side by side in order to tell the difference.
Oh, and if you want to know what the rare version is worth, I recommend
a visit to your local library where you can check out the buy/sell ads
in the back of the various philatelic periodicals. Don't ask me, because
I don't care what they're worth, and as a consequence, I don't know what
they're worth -- I collect for pleasure, not for financial speculation.