I recently had some trouble getting ssh public key forwarding working with Capistrano and my Mac.
I thought it might be helpful for others if I documented it 
It’s actually pretty simple.
In your capistrano recipe you need to explicitly state you want to use forwarding:
ssh_options[:forward_agent] = true
and from the command line:
$ ssh-add -l
Hope that helps somebody! (other than me 
)