How do you decide that stocks are P/E is too high now as opposed to it being too low in the past. The us equity market has historically had much higher returns

How do you decide that stocks are P/E is too high now as opposed to it being too low in the past. The us equity market has historically had much higher returns than the rest of the world. There's no reason to believe that could continue forever. At some point world markets will reach an equilibrium.