In the beginning of the last paragraph of that two page PDF you linked to, Bostrom writes:
"If we are in a simulation, could ever know for certain? If the simulators don’t want us to find out, we probably never will. But if they choose to reveal themselves, they could certainly do so."
So, I understand this as the author saying we (scientists, physicists and the like) can't prove this is a simulation from our end. Without that top-down revelation, I think the most we could get is an accidently true belief...which is short of knowledge (which may also depend on if we could agree on what knowledge would entail).
It's a fun thing to think about, and it makes me wish I knew more about Philosophy of Mind.