Your skepticism is great. Happily, Building Tomorrow has a great track-record, and has made being transparent one of their core principles. You can read more about it here: http://www.buildingtomorrow.org/zeta/about-us/transparency/
You can also check out third parties who've reviewed them, http://givinglibrary.org/organizations/building-tomorrow and http://www.guidestar.org/organizations/56-2614329/building-t...
Thanks!