ssh-keyscan -t rsa YOUR_SERVERs_IP_ADDRESS_OR_HOSTNAME
Output:
# YOUR_SERVERs_IP_ADDRESS_OR_HOSTNAME:22 SSH-2.0-OpenSSH_5.3
YOUR_SERVERs_IP_ADDRESS_OR_HOSTNAME ssh-rsa AAAAB3NzaC1yc...
Check with the several type of the keys:
ssh-keyscan -t rsa1,rsa,dsa,ecdsa,ed25519 YOUR_SERVERs_IP_ADDRESS_OR_HOSTNAME