Ah, yes - there it is (in KeePass/KeeAgent, under `Advanced`, click on the private key file (*.ppk) and then on Open > Internal Viewer).

> PuTTY-User-Key-File-2: ssh-rsa..Encryption: aes256-cbc

Indeed I seem to have used Puttygen in the past.

For keys from Linux ssh-keygen, the private key starts with:

> -----BEGIN OPENSSH PRIVATE KEY-----

and the public key starts with

> ssh-ed25519