Legally a charity must not use more that a percentage for administrative expenses. I don't know, but it is something like 15-20%. The rest is supposed to pass on to the work.
As we should all be aware, charities are being exposed using the money for everything from trips to the Bahamas to sex with prostitutes, and even political activism, which is supposed to be illegal. Your best bet is to locate a small neighborhood charity that you can check out easily. These don't get enough revenue to waste. If you donate to a church, it should be the one you attend, or one you respect and know has good morals.