How can you....

The only way I can think of is to check with the Better Business Bureau. Just go to bbb.org and check on a business. If they are not listed, you can have them check them out for you.
I know what you mean about handing money over. Especially on the computer. You never know who is on the other end!
I usually only donate to organizations that I am familiar with. Being that I got Maddie from FBR, I feel pretty comfortable with donating to that rescue. Although, I really never know where my money is going. I guess that is where the trust issue comes in.
Other than that rescue, I really only contribute to a couple of national organizations. Of course, when I go to petco or petsmart and see the people from the adoption agencies sitting out front with their animals and their jars, I always have to put a little something in their jars.  :wink:

This is what the rescue I got Samson from says about themselves
-- We are a 501(3)c nonprofit organization approved for rescue in Alabama and Georgia. We are run entirely by volunteers dedicated to the rescue of unwanted, neglected, and abandoned boxers. We are always in need of qualified homes for life for our dogs, transportation, temporary foster care, monetary donations for veterinary care, high-quality dog food and other supplies.
Boxers come to us from shelters, as strays, and owner surrenders because of death, divorce, or relocation. They are housed by foster homes where they are temperament tested and seen by a veterinarian for shots, heartworm testing, and spay or neuter. ---

 If i remember correctly you want to make sure they do state they are non-profit and like Barb said, also a member of ABR. There's a boxer rescue link on the front page, all of the rescues listed are approved and I would have no problem donating to any of them...