1) do an "info:www . theirsite . xtn/their-linkingpage.htm" search to check that that page is indexed, and then 2) check the cache for that page to be sure that your link is on the version of the page that Google sees.
AFAIK this is the only way to be 100% sure that Google is seeing what you're seeing.
You're 100% right, it shows that Google has indexed a page. But it does not indicate if the link is being used to rank a page. It does not indicate if the link is only partly credited. It does not show if the link does not count at all.