When I decided it was time to become a volunteer, I found the rescues that operate in my area and pretty much stalked their websites on a daily basis

. Seeing how often updates are posted, what dogs they take in and how quickly (or not) they get adopted was helpful. The rescue I chose is approved here on BW and I actually made contact with them by posting here. My first meeting was at an outreach event where I got to meet several of the local volunteers, see how they operate and decide if it was the right fit for me. It's been over a year, so I think I'll keep them!
You can look up a group on this website
Search for Charities, Online Version of Publication 78 to make sure they are a registered charity. You can also ask about minutes from their board of directors meetings. Ask what a volunteer is expected to do and how information is broadcast out to the members. Are any of the members paid for their time or is truly an all volunteer effort? What happens to the dogs that don't get adopted? Is the rescue dedicated to making sure the dogs are safe and treated well the entire time they are in their care? How are they funded and how do those funds get spent? Do they put any limits on medical care while a dog is in foster? What expenses are the responsibility of the foster family? Call your local shelter and ask them which rescues they have dealt with and what their experiences have been. Ask about the approval process for new adoptive homes - how thorough are they?
Thank you for helping whichever group you decide on and good luck!