Actually, I think most of the measure of people having Death Notes is... in Death Note itself. Thus, if I had a Death Note, I would logically conclude that the most likely explanation is that I myself am a character in Death Note. Not in the original manga, of course, as I read that and I know I wasn't in it, but likely in some spin-off. I could easily see myself as a character in some sort of Death Note video game/simulation.

I am on the fence about the Simulation Argument, but even so, this is exactly the kind of thing which is strong evidence that I am a fictional character in a simulation. Getting a Death Note? That's the kind of thing that only happens in stories!

(OK, it is true that I should keep in mind the possibility that I simply have gone insane. That is also a reasonable explanation. But it is far from the overwhelming certainty that you are implying.)