Thanks!
A method of telling it’s a SHA1 key, which I’ve just worked out from Google searches, is that “digest algo 2” will appear in the output of
$ gpg --list-packets keyfile
[root@foo:production:~]$ gpg --list-packets tenable-2048.gpg | grep -i "digest algo 2"
digest algo 2, begin of digest 28 63
digest algo 2, begin of digest 81 4f
[root@foo:production:~]$
and if you look at RFC 4880 - OpenPGP Message Format it says that algoritum number 2 is SHA1. Which seems like a very obscure and cumbersome way of finding out. Did you do that or did you do something more straight forward?