Let's say I'm an undergrad...Let's say the research the lab I'm in does is already what I want to study in grad school. Can I simply ask the PI, who is already familiar with my ability to supervise my PhD or is that not a good idea? What if I network with other labs that do similar/related research and just casually ask?

I'm just curious, because I already know what I want to do, I hate school (I honestly just want to chill out and just get B's and focus on research) and I definitely don't want to care about scoring as well as possible on the GRE.