I fall over this every so often. I have the private key file but would either have to trawl servers for authorized_keys files to get the public password or remember how to obtain the public key from the private key.
Time to document it here so I don’t have to hunt for it with Google again.
For an RSA PEM format public key
$ openssl rsa -in private.key -pubout
-----BEGIN PUBLIC KEY-----
-----END PUBLIC KEY-----
For an SSH putty friendly version
$ ssh-keygen -y -f private.key
ssh-rsa AAAAB3NzaC1yc2EAAAABJQAAAQE ...