This is likely to be an issue where there's a huge variety within sub-subfields. These days I'm mostly working on fusion categories and algebraic aspects of subfactors. Within the broader field of quantum algebra and quantum topology, these are a much smaller subfield than say Khovanov-style homology theories/categorification of quantum groups. As such, I don't have to worry as much about competition, and basically know who would be likely to be already working on any particular problem. In more crowded fields this is going to be much harder. This is one of the things I like about working on a less crowded topic.