Are you talking about the base stats? If so, simply visit the Pokemon's Pokedex page on this website. There will be an option you can choose which will show the new stats and abilities of the Mega.
If you are talking about the actual stats of Pokemon, then you cannot view them outside of battle. This is probably because Mega Pokemon technically don't exist outside of battle. The only way to see these stats is to visit the stats portion of the summary of a Pokemon while it has Mega Evolved in battle.
