If you read enough literature and try to implement enough stuff yourself, you will eventually run into a question that you cannot immediately answer.
Often when I read a paper in a subject new to me, I realize that I do not understand something and make a note to study that question further. A lot of times, that question is already answered somewhere. I find an answer to it --- either by asking a colleague, searching for papers or books on the subject, searching the internet, or even just by trying to solve it myself --- and I then just move on once I get an adequate answer. But sometimes, even in subjects that I am an expert in, I write down a question that I cannot find a good answer to at all. Then I know that I have something that is worth researching further.
Often when I read a paper in a subject new to me, I realize that I do not understand something and make a note to study that question further. A lot of times, that question is already answered somewhere. I find an answer to it --- either by asking a colleague, searching for papers or books on the subject, searching the internet, or even just by trying to solve it myself --- and I then just move on once I get an adequate answer. But sometimes, even in subjects that I am an expert in, I write down a question that I cannot find a good answer to at all. Then I know that I have something that is worth researching further.