Enter beginner. If you stomp on everyone, stop entering beginner. It's fine if you accidentally sandbag once or twice to figure out where your level is. If you do it repeatedly to pile up wins, then you suck.
I just fought my first nogi tournament, and lost in the semis to a guy with 16 professional MMA bouts, and like 12 years of wrestling. I actually should have won, and was ahead on points, but misread the time on the clock and made a dumb mistake. I was a little annoyed at first, but then he came over, was really nice, and explained that he had never done a submission tournament, and wanted to see if his BJJ was good enough to fight advance. Fair enough, I would have done the same thing. Point is, don't worry too much about that as you first start to compete. Figure out where your level is by fighting, and enter into divisions accordingly.
BTW, this is why I think nogi tournaments need more divisions. Beginner, Advanced, and occasionally Intermediate? Gi has White (often beginner White and Advanced White), Blue, Purple, Brown, and Black. I understand that fighters with no BJJ rank need to compete at higher levels (wrestlers, judoka, sambo fights, etc.), but just allow people to register at any level, using the belts as guidelines. You could call them Beginner, Intermediate, Advanced, Expert, and Elite or something if you want instead. Like I said above, people will easily figure out what level they belong in based on their results. I always thought it was ridiculous to see blue belts enter into advanced nogi and fight black belts.