Is there a mod to see/reveal map before starting?

You can use the IGE mod on Steam. It allows you to toggle fog and change tiles/add units/etc. You can use it by loading it as a mod in-game, then there should be an "IGE" option in the top right corner after you load your game. It's not perfect, but it's the closest thing to what you're looking for.
 
While IGE is a great tool to use, the user tends to cheat while using it (like me). If you want to play with a clean Civ V experience, I suggest you to get Reseed! instead.
It has the same functionality to reveal the whole map, and you can even regenerate the map and look at the preview of the results first.
 
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