I think this may work... just do a GET https://spreadsheets.google.com/feeds/worksheets/key/private/full
where *key* is the entry from gd:resourceId spreadsheet:*key* that you got from docs.google.com. The return feed will contain entries corresponding to the worksheets of your spreadsheet. If you really want to find the Spreadsheet API key, then just do a GET https://spreadsheets.google.com/feeds/spreadsheets/private/full then find among all your spreadsheet entries the one where <link rel='alternate' href='https://...'> matches the key you found from Docs API, then get the <id> of that entry. Hope this helps.
