is exist way to take all external walls of building
an example
but im not shure that its method is alwayes correctly work…
Real building is more difficult of course 
properly set walls to interior or exterior in their function (type parameter), then sort all walls by that parameter.
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make a big wall outside the perimeter of the building and place a room called “outside” in between and see if the wall in question is roombounding for the room “outside”
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Like Marcel said if you create an external room around your building at the same height of your building you can find the walls that bound this.
This script is just a starting point I have only tested it on a single storey building it might need adapting for multiple stories.
finding external walls #5.dyn (19.6 KB)
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I just came across this and have a thought.
Why don’t you get the room boundary curves along with the wall center line. The wall centerline that has two curves next to them are internal walls while ones that have one boundary curve is and external wall.
Best of luck!
Any situation with parallel walls adjacent to each other would classify as exterior in that method, which may be rare for your projects would be frequently be an issue for the ones I used to do.
The best methods are to use an exterior room as noted here, or to use the BuildingEnvelopeAnalyzer class of the Revit API.
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You can use a the point along the mid curve and and get the distance from the centerline and make sure it is within the distance of the wall thickness.
I will definitely checkout the class you posted and try to understand it.
I have a question regarding the rooms. Is there anyway use the room class and get boundaries without manually place a room in revit? I have been searching it all and all the room automations are through defining the data of the rooms and then place them manually… Would be good to know if the it is possible.
Best to start a new topic as this one is quite old and your topic isn’t directly related, but the answer is… yes, but usually you’ll want the rooms anyway so I find it best to make them.