From a practical standpoint they were pretty well versed b/c the payload required very specific and distinct handling such as loading over special pits dug into the ground, arming the bombs during the flights, taking into account how much flight characteristic would change at the moment of dropping the bomb (by instantaneously shedding a significant proportion of the plane's grosse weight) and by receiving instructions for the path to be taken in the immediate aftermath to avoid the blast and confirm / observe detonation. Not to mention the support aircraft that flew along to make measurements and observations as well. So from a practical standpoint they were well briefed on what to expect, thought that's not to say they weren't still surprised awed by witnessing the detonation and blast. From a technical standpoint, of course, I think they knew little more than that this was a new kind of extremely powerful bomb. Check out the documentary "the Men Who Brought the Dawn" which currently runs on the Smithsonian Channel in the US occasionally, and on youTube as needed. It includes interviews and exactly the type of material you're looking for.

I'm not a historian by any means, but there's a great autobiography by the only man to fly both missions, Major General Charles Sweeney called War's End. Fantastic read, really simple in its layout & can be finished in a weekend... I've personally bought the book a half dozen times because I keep loaning it out. He goes into the whole process that led up to the final bomb drop and how much time they put into their training, and though they weren't certain it would be an atomic bomb they were dropping until well after their training started, they knew it was something significant & could change the course of the war. Highly recommend.

On a fun side note - the program they were under was called "Silver Plate" (as in anything these guys need, they can have on a silver platter). One of the auxilliary guys who assisted the flights got caught up in a drunken brawl & told the local sheriff he couldn't be arrested because he was part of Silver Plate. The sheriff started asking questions & this guy ended up riding out the rest of the war in Leavenworth. They were that serious about security.

All of the crew members had trained with dummy bombs known as "Pumpkins." These were basically simplified Fat Man bomb casings filled with high explosives (they could be filled with nearly 3 tons of HE, so even without an atomic charge it was a considerable conventional bomb). They had practiced dropping these both under experimental conditions as part of Project Alberta, connected with the Silverplate project of producing B-29s that could carry and drop atomic bombs, and had even dropped many of them on Japanese cities in trial raids. (Which produced some pretty amazing pictures — the closest thing we have to photos of what the atomic bombs would have looked like, going down.)

So they knew the ballistic weights and the secrecy and the fact that the bombs that were being loaded in were "Special."

Col. Paul Tibbets, who captained the Hiroshima mission, knew the whole story. Several other members of the Enola Gay did as well, because they were involved in the post-takeoff assembly of the bomb (e.g. Capt. Parsons). The other crew members were not fully in on the secret but every account emphasizes that they had a pretty good idea — one of them supposedly asked Tibbets if they had something to do with splitting atoms, which he replied in the affirmative. Other than Tibbets and Parsons I don't think anyone on the Enola Gay probably knew exactly how large the explosion would probably be.

By the time of the Nagasaki mission, of course, everyone knew what it was — the fact that the US was dropping atomic bombs on Japan was no longer a secret.