I've heard the people say "Amen" to the epiklesis only here in America, at an OCA church and at an Antiochian one. Never back home, never in a Greek church.
It depends on what Greek Church you attend. Both the one I grew up at and the one I serve currently say the epiclesis aloud with the people responding with the "Amen." At the Theological school, it was said "mystically," with only those in the altar responding (deacons if there were any, other clergy, etc.).