Found something. On a LinuxMint machine, under System Settings, I changed the default browser, exited, then ran "find ~ -mmin -1 -type f", which showed me the 2 files that had been modified in the past minute. ~/.local/share/applications/mimeapps.list was a hit, it shows several default apps including the mailer. I can run "grep mailto .local/share/applications/mimeapps.list" to display the default mailer, or "grep http .local/share/applications/mimeapps.list" for the default browser, etc. It works on the Debian box too. I'll use it until it breaks.