Polvo.

[Jane]

Do you say Estoy hecho or hecha polvo?
And how do you say something like this in Spanish `You could´ve fooled me.´

[Rusty]

If the subject is female, it's hecha. If it's male, hecho. If the subject is plural, like 'we', it's estamos hechos polvo.

You could've fooled me -
¡No me digas!
¿Sí? No me había dado cuenta. (con entonación apropriada)

There are probably many other ways to say it.

[Iris]

Rusty's explanation is perfect, but I wanted to say hello.
Another translation for you could have fooled me is casi me lo trago.
Nice to hear from you.

[Jane]

Thanks for the warm welcome Iris,
He estado aprendiendo mucho.

[Tomisimo]

Another option for you could've fooled me! - ¡casi me lo creí!

[sosia]

more options
-Estoy hecho polvo. Estoy destrozado.
-¡por poco me lo creo!
saludos