I had to check which SSH key is stored on Github. But there only the fingerprint is shown, not the public key. The following command gives me the answer.
cd ~/.ssh ssh-keygen -l -E md5 -f id_rsa # 2048 MD5:ab:cd:ef:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff michael@malura.de (RSA)