I am admittedly an undergraduate so I do not have very much experience yet in finding resources.
When I have an idea of something I would consider worthwhile to research (in my case for a proposal to a supervisor about an undergraduate research opportunity), assuming I do extensive research online to see what has been found out about the subject and end up concluding that what I want to look into has not been investigated yet.
How can I ensure that it actually hasn't, and it isn't just a case of me not finding the information that is already available somewhere?
Several years down the line, I'm now be able to answer this question myself: you can't exclude it, and this isn't limited to academic topics. People will disclose what research, products, startups or concepts they are working on at conferences, competitions, social events,... but you can never know whether they're working on something and intentionally not disclosing it. Internet research is the closest you can get, supplemented by asking people who know the field you're interested in.