If these links really haven't been found by Google in 6 months, then it's likely that those linking pages themselves don't get spidered, for whatever reason, and that those links wouldn't help you.
When you say you can't find the url in the serps, I assume you're talking about the url of the linking page. You don't say, though, how you're searching for it. As martinibuster says, Google knows a lot more about your backlinks than what it shows with the [link:domain] operator.
You could do two other kinds of searches for the linking page and check the cache to see if a link to your page has been indexed.
- one is to do a [site:domain] search for the linking page. If it's a very large site, you might not be able to find the page that way.
- the other is to search for a unique word string on the linking page to see if the page ranks on Google for the string. Pick a string of a half-dozen to a dozen words from the page, put it in quotes, and search on Google. If the page doesn't come up, it's likely the page itself is not indexed in Google, and that your link isn't either.
If it is indexed, and your link is in the cache, it still may be a while before you see the effects of that link, but you know that Google's got it.
Searching for an exact string on the linking page, btw, is one way to evaluate the page in the first place.