Advice for early grad student: how to professionally present tentative research idea(s) before one has chosen a topic
You should be honest for sure. If you don't know something, do not pretend you do because you may face a person who really knows what you try to talk about and then, if you have only a superficial grasp of the subject, you are toast. Just say straight that you've heard that X is an interesting subject, that you have already read , that you want to learn more about it and, perhaps, try a research project in that particular area, and that you have this and that idea and wonder what can be made of them. That will get you the warmest response you can expect from the person you are talking to ("the warmest" doesn't mean "warm", by the way) and you may get some good advice or help this way from a "complete stranger". 

I admit that many people fail to understand that knowing and openly delineating the limits of one's knowledge and powers is "a professional behavior". Nevertheless, those whose respect you should really earn during your academic career (whether they are your professors or your fellow students at the current moment) will judge you from this standpoint. The opinions of the rest may make a lot of difference on the short run but none on the long one.