Polvo.

Polvo.
 

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

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.

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.:):):)

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

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

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