I have an old save, I've saved everything, the whole .minecraft folder. I want to know the exact version of that minecraft save, I think it's 1.0, but I wanna be sure. Where do I find the version of the game? Is it somewhere in minecraft.jar?!
Have a dig through all the folders- open files in notepad. It'll be there somewhere.
If you have access to the old (pre 1.5) launcher- run Minecraft with that launcher to create a .minecraft folder. Switch the newly created .minecraft for a copy of your old instance and launch MC. If it boots just look on the f3 screen.
If its a modded instance- look at the version number of mods, and see what MC version they need.
If you have screnshots- check the date against MC releases. (or look at 'date modified' on the files in the .minecraft)