How to compare different SSH fingerprint (public key hash) formats?

ssh

# ssh -o "FingerprintHash sha256" testhost
The authenticity of host 'testhost (256.257.258.259)' can't be established.
ECDSA key fingerprint is SHA256:pYYzsM9jP1Gwn1K9xXjKL2t0HLrasCxBQdvg/mNkuLg.

# ssh -o "FingerprintHash md5" testhost
The authenticity of host 'testhost (256.257.258.259)' can't be established.
ECDSA key fingerprint is MD5:de:31:72:30:d0:e2:72:5b:5a:1c:b8:39:bf:57:d6:4a.

ssh-keyscan & ssh-keygen

Another approach is to download the public key to a system which supports both MD5 and SHA256 hashes:

# ssh-keyscan testhost >testhost.ssh-keyscan

# cat testhost.ssh-keyscan
testhost ecdsa-sha2-nistp256 AAAAE2VjZHNhLXNoYTItb...
testhost ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQC0U...
testhost ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIMKHh...

# ssh-keygen -lf testhost.ssh-keyscan -E sha256
256 SHA256:pYYzsM9jP1Gwn1K9xXjKL2t0HLrasCxBQdvg/mNkuLg testhost (ECDSA)
2048 SHA256:bj+7fjKSRldiv1LXOCTudb6piun2G01LYwq/OMToWSs testhost (RSA)
256 SHA256:hZ4KFg6D+99tO3xRyl5HpA8XymkGuEPDVyoszIw3Uko testhost (ED25519)

# ssh-keygen -lf testhost.ssh-keyscan -E md5
256 MD5:de:31:72:30:d0:e2:72:5b:5a:1c:b8:39:bf:57:d6:4a testhost (ECDSA)
2048 MD5:d5:6b:eb:71:7b:2e:b8:85:7f:e1:56:f3:be:49:3d:2e testhost (RSA)
256 MD5:e6:16:94:b5:16:19:40:41:26:e9:f8:f5:f7:e7:04:03 testhost (ED25519)

Only answering how to view local keys, which is also visible on the other answer but could be missed. On Ubuntu 19.04 version at least, SHA256 is the default format for ssh-keygen:

$ ssh-keygen -lf ~/.ssh/id_rsa.pub
2048 SHA256:CxIuAEc3SZThY9XobrjJIHN61OTItAU0Emz0v/+15wY user@host (RSA)

But you can explicitely specify SHA256 of course:

$ ssh-keygen -lf ~/.ssh/id_rsa.pub -E sha256

If you want to view the MD5 instead:

$ssh-keygen -lf ~/.ssh/id_rsa.pub -E md5
2048 f6:bf:4d:d4:bd:d6:f3:da:29:a3:c3:42:96:26:4a:41 user@host (RSA)

Which, by the way, is the format used by GitHub on their list of SSH keys on your account. For details: $man ssh-keygen.