Spoilers if you didn't finish the game...
They didn't really. They only know how to build it, and that it can release / produce a lot of energy if "fired". The plans were adapted to be understood by all (any race). From the plans, they knew the catalyst was still missing, and, with Shepard's help, they find out from the Prothean VI that the Citadel is also part of the plan. At the very end of the game, Hackett mentions that the Crucible isn't doing anything, and that it must be something on Shepard's end meaning they still probably didn't have a full understanding of the weapon and how it exactly worked. JohnMD (talk)
Like it was said above: They did not. Hackett even compares deploying the Crucible to the first time an atomic bomb was used in WWII. Much like the Crucible could scorch the galaxy, the scientists speculated that the deployment of a nuke could ignite the atmosphere. They even made bets about the success of the nuke IIRC. LeoJo (talk)
Indeed, the entire plan was made on a hope and a prayer. It was all they had left. If the Crucible hadn't worked, they all be instantly killed being that all the major military strength was on and around Earth. But at that point, they were all facing imminent death anyway - it would have just come a bit quicker. Xelestial (talk)