The easyest way to tell if you have a 1957 283 is to look for the "side" motor mount bolt bosses (the 3 raised nubs on both sides of the block that the motor mount bolts to). 1957 is the only year 283 that does not have them, they still used the front mount. ALL SBC's from 1958 and up have side mounts.
As for value, if it's a true truck motor it's worth about what any running SBC is worth. If by some chance it's a car engine the value could go up. The stamped number on the passenger side tab in front of the head will tell the tail.