Generally speaking, I'll always check the charity out if they seem sincere.
First strike is that they're calling. Charities pay companies to call. You donate $20, and $16 dollars of your contribution go to operational costs, such as the charity your giving to paying the company that is calling you.
IMO you always need to research the charity, find out what kind of organization it is, and how much money given is actually used to "help".
For some reason, executives of charitable organizations who make 6 figures easy in salary has always rubbed me the wrong way. I try to give as local as possible.