[AngelicaDeAlquezar]

"Podías/pudiste haberme engañado", depending on what you mean.

Simple past is used when someone has fooled you once, but you're not in disposition of letting them fool you again:

Pudiste haberme engañado en el pasado, pero ahora ya te conozco bien.
You could have fooled me in the past, but now I know you well.


Imperfect is used when someone could have fooled you in the past, but didn't (either because you caught them or because they changed their mind).

Podías haberme engañado diciendo que estabas enfermo, pero te agradezco que me hayas dicho la verdad.
You could have fooled me saying you were sick, but I appreciate that you have told me the truth.


An alternative to Poli's example:
Jimmy told me he was sick, but he could have fooled me.
Jimmy me dijo que estaba enfermo, pero pudo haberme engañado.