I'm recently becoming aware I might be one, though it's taken me years to realize it. I got a prestigious award no one else in my department had ever gotten during my second year; I have external funding, something else that's rare for students in my program. I work in an interdisciplinary lab, and we've had a number of high profile visitors come to see my project. My lab work is the project that I proposed in my second year, which is also my dissertation research. This project is actually a major area of research for my lab, which is I guess kind of unusual since either professors or PIs start propose and start projects rather than students.

If you ask anybody other than my advisor, apparently I'm a rising star. I still don't feel like it, and I heavily suspect I'm not, but it seems unlikely this many people would be blowing smoke up the ass of a lowly grad student if I hadn't done something of note.