The one with the rounded roof is the indestructible.
In case you have trouble remembering, remember that each air base has at least two bomber hangars, but only one indestructible hangar. The indestructible hangar will be the one that is different.
A GV base or a port will have one indestructible hangar, and no bomber hangars.
1ijac (One-Eye Jack) made some excellent maps which have all the hangars and guns marked. I forget the address to his web page, but I have them on my page at
http://www.kentwoodstation.com/Maps.html.
Hope this helps.