How to test map vision / how to open bnet maps in map editor

Go into the Map Editor and select File -> Open in the Menu. Then click on the Tab "Battle.net" on the left, Log In with your SC2 account details and then you can open any map that is published on the Battle.Net server, including Blizzard maps.

You can also play one of the YABOT or SALT maps with a friend. YABOT allows for a restart of the game on-the-fly and SALT allows to save gamestates and reload them, which allows you to test stuff quicker since you don't always have to open up the map again.

If you have a replay with the scenario and are not certain if the opponent saw it you can select his vision and check.

But the easiest way is to open the map in the map editor, chose the position you want to check, place neurtral building and act as if you were your opponent.

If you choose "Very Easy" AI you have about 10 minutes to check with your initial workers. That should be enough to check the vision.