I have been searching around a lot on this topic, but did not find a single source of correct information. How can one find out if a particular feature one is using in one of its products is patented or not?
I'm particularly interested in finding out if the iTunesU note-taking feature is patented. Any information on whether this feature is patented or not would be helpful.
This is a very difficult questions to answer, and a company wanting some kind of answer to this could easily spend $10,000-$50,000 on patent attorneys to do the research.
The first question is whether Apple owns a patent that covers it. Companies are supposed to "mark " patented products with the number of the patent. This makes the most sense when you are buying a physical product such as an iPhone. For an online product, there is not one obvious place to put the patent number and patent marking may not even apply when there is not a physical product (this is too complex to go into here).
Even if Apple doesn't have the patent numbers marked somewhere on the product, it is easy to search all patents owned by Apple to find ones that would be relevant. However, Apple's patent applications may not be publicly available so Apple may not have a patent on it now, but could have one next week or next month.
But just as important is whether a third party has a patent that covers it. This is a much more difficult search task because of the sheer volume of patents and because the same concept can be expressed in a variety of different ways so you may search using the wrong keywords. You could easily spend hundreds of hours reviewing patents to find the needle in the haystack.
In summary, unless you are really lucky, you can't get an answer to your question without spending lots of time and/or money.
Most importantly, why do you want to know if this feature is patented? Unless your company is hugely successful, Apple will not waste its time asserting any of its patents against you. If your company is hugely successful, then you should have your own patents and a budget to defend yourself.