How can the dead know anything? At death the circuits are turned off. Like when you turn your computer off, the RAM loses power and everything in memory is gone. The brain is more like an EPROM, where instructions are remembered after power is turned off and when power is turned back on, it can continue with the instructions that it has in its programming. That is what it is like when a person is resurrected.
But people people haven't been resurrected since the apostles day so none of the EPROM brains have been repowered. So they dead know nothing.