The ssh-keyscan command with the -F option can be used to determine if a known_hosts file does or does not contain a certain hostname. For example, let's say /home/john.doe/.ssh/known_hosts contains the following.

server1.example.com ecdsa-sha2-nistp256 AAAAE2VjZHNhLXNoYTItbmlzdHAyNTYAAAAIbmlzdHAyNTYAAABBBHXIAfxTJu1y7QdQNrk6xh41FH1fqIVbG2Skvhx49PDfvm5pCdiyHqPP0pcyM7UGJOAPazKNENGZtqmnH8CUDo0=

In this example, the following ssh-keyscan command should state that the known_hosts file does contain "server1.example.com".

~]# ssh-keyscan -F server1.example.com -f /home/john.doe/.ssh/known_hosts
# Host server1.example.com found: line 1
server1.example.com ecdsa-sha2-nistp256 AAAAE2VjZHNhLXNoYTItbmlzdHAyNTYAAAAIbmlzdHAyNTYAAABBBHXIAfxTJu1y7QdQNrk6xh41FH1fqIVbG2Skvhx49PDfvm5pCdiyHqPP0pcyM7UGJOAPazKNENGZtqmnH8CUDo0=

On the other hand, the following command should return no output, meaning the known_hosts file does not contain "bogus.example.com".

~]# ssh-keyscan -F bogus.example.com -f /home/john.doe/.ssh/known_hosts