Before you go and hold a garage sale for a charity, you want to make sure you're not supporting a scam or a fraud. Determining if a charity has merit is a process that requires you to do a little hard investigation work, and sometimes to ask for the answers to questions people are refusing to acknowledge. Most of the time, you won't need to worry about a charity's validity -- you can do some research online with the name of the organization plus the word "scam" to make sure there aren't any big red flags out there. Other times, though, with smaller or newer organizations, you may need to do a little digging.
Start off by reading the literature put out by the charity. Of course, this is often exaggeratedly good in what it says about the organization, considering that they're the ones putting it out there, but sometimes you can read between the lines and notice a darker backstory. Most charities, though, are upstanding and wonderful, so don't go into your research assuming they're scamming the generous public!
If you still have your worries after reading the statistics and testimonials, try talking to others who have donated. Past donors may keep track of where their money goes, if they're concerned about how it's being used, so they may have some information for you that the organization itself is unwilling to release. It's not likely that you'll be able to easily find a list of past donors, so you could have to just ask around via word of mouth until you track someone down.
Feel like you should ask within the company. Instead of going to the higher-ups, who have their company's best interest to think about, try talking to basic-level employees about the procedures and policies of donations to the organization. While they might not have all the answers, they may also be less concerned with keeping any discrepancies in the dark, since they do not have as much of a vested interest in the organization.
Though the above methods should help you determine whether or not a charity is worth supporting, you can see about going in yourself to take a look at the organization and what it does on a daily basis. This is great for practical charities, such as those that feed the homeless and hand out hygiene products, or those that assist battered women. You may even find yourself giving some of your own time spontaneously!
Check out GarageSaleCow.com for a great place to list your charity garage sale, once you've settled on the perfect organization to support.