Some of us are definitely robots and it's hypothesized that some of us aren't.
It's hard to tell the difference, though, because the robots are known to be hard-coded to believe they aren't robots.
My rule of thumb is to try to evaluate how strenuously a candidate denies being a robot. If s/he doesn't seem to care too much, s/he may in fact be human.
But I admit haven't found a decisive test yet, so there's no way to study the hypothesis rigorously.