Use ssh-add -l to list them by fingerprinttheir fingerprints.
$ ssh-add -l
2048 72:...:eb /home/gert/.ssh/mykey (RSA)
Or ssh-add -L to get the full key in OpenSSH format.
$ ssh-add -L
ssh-rsa AAAAB3NzaC1yc[...]B63SQ== /home/gert/.ssh/id_rsa
The latter format is the same as you would put them in a ~/.ssh/authorized_keys file.