The first option is to always do some keyword research.
Another option is to type different keyword phrases on Google (without hitting enter or search) and see what Google brings up as suggestions, you can do the same process on Youtube as well.
Another option is to go to a popular youtube account and sort the videos by popularity, mostly the videos are popular for a reason, do the same thing on different youtube accounts. Check if the number of views is greater than the number of subscribers on that channel.
If the number of views is the same, nearly the same or less than the subscribers then it's not a popular topic and that means the video got a lot of views because the youtube account has a lot of subscribers. The difference needs to be big and obvious.
For instance, if the youtube account has 20000 subscribers and the video you're looking at has 15000 to 25000 views then it's not a popular topic. The video probably got many views because the youtube account has many subscribers.
If someone has 20000 subscribers then a popular video will be a video with 45000 views or more.
Another option is to go to popular blogs, most blogs have a tab on the sidebar that shows "popular posts", those topics are popular for a reason. This means that the topics are mostly what people want more knowledge about.
Same applies to forums, check which topic keeps coming up in forums. The more frequently the questions is asked then you know it's going to be a good topic to create a video about.
You can also type your keyword in question websites such as Yahoo Answers to check what people are frequently asking when it comes to your niche, this will also help you come up with some good topic ideas.
Hope that helps
