"@Commenter: " . . . miss a typo after pouring over a document . . . " I'm not sure if your use of 'pouring' here is an intentional demonstration of the point, or not - but note that it should be 'poring' instead. "
Image by the_munificent_sasquatch via FlickrI think having good grammar and writing well is a really essential skill, but sometimes the nuances of a language can be tricky to master. (Especially for non-native speakers; you have my sympathy.) This example above is a good case in point: which is right? Poring over, or pouring over?
One simple way to check is to use google. Open a search, and type "pouring over" and note how many results you get - today for me was 398,000. Then type "poring over" and compare the results - 624,000. (Mind those quotes, they're important for search.) Based on these numbers, I can see that "poring over" is correct.
Not everything is amenable to a simple popularity contest. Check out a book or two for a more thorough understanding of grammar. I think grammar generally is adequately judged by common usage, because grammar arises from conventions. But be warned: acting as the grammar enforcer at work or around your friends won't win any popularity contests. Another good resource to check and see if an expression you're using in writing is a problem is Wikipedia's page on common misspellings/grammar and Misc.