How can we know if a charity is legitimate so we won't be throwing our money away?

Clark Howard (a financial/consumer wiz I like) has some good basic recommendations

Personally, I prefer to look into the charity and find out:
-The percentage of donation that actually gets used (not for admin)
-How much the president of the organization earns annually
-If they need money or things more (in many cases, the animal shelters I donate to need things like food or blankets ASAP at any given time)

I tend to keep my donations very local. That way I can see my dollars at work and can see what needs I can help with.

In the Netherlands we have a ‘hallmark foundation’ that oversees charity’s intentions, goals, and finances. Translated Wiki
Maybe there is such an organisation in your country too?

You might try www.charitywatch.org

As said above, do some research. There are organisations in Australia that also monitor what charities do, how much they spend on admin etc. I want to be sure I am donating to organisations that are efficient and make the best use of my hard-earned money.

Play it safe.
Don’t give anything.
Or wikipedia that shit up.

I suggest Charitynavigator.org

Charity Navigator is really the first place to start. They make sure that the charity is spending money on actually helping people, not on CEO bonuses and ineffective administration.

But, for stuff in other countries, you really do have to do some research. Figure out what’s going on in the region, and what you want to be done – not just, something, but specifically what. A lot of really good charities by Charity Nav’s standards are going to be NGOs of the paternalistic, White Man’s Burden variety. It’s really a lot easier to navigate not just the finances, but the politics, when it’s local or at least in the same country you live.