I believe ssh-keygen -t ecdsa -b 521 pub keys will have ecdsa-sha2-nistp521 in plaintext at the start. I don't know how to tell from the priv key. And I think

I believe ssh-keygen -t ecdsa -b 521 pub keys will have ecdsa-sha2-nistp521 in plaintext at the start. I don't know how to tell from the priv key. And I think converted key pairs in Putty format (.ppk) will have PuTTY-User-Key-File-2: ecdsa-sha2-nistp521 in plaintext. For Pageant you should be able to select view keys from the system tray icon context menu and it should show the key type in the list. For ssh-agent I think ssh-add -L should list the public keys (with key type) in the same format as the authorized_keys file I'm not an expert, so if anyone is please correct me where I'm wrong!