The goal of action, critical theory argues, is emancipation. Reason and action must come together. Action is not simply based on abstract norms or principles, no matter where they came from. Reason is not simply the ordering of reality.
You can tell if a system of thought is ideology if either action or reason fall into the categories just described. So, what's emancipation?
For Marx, recall, it was the movement of history toward increasing class conflict. Ultimately, the oppressed are emancipated because their struggle yields a new political system.
Critical theorists do not take this solution. They see the problems of totalitarianism in the Soviet Union. There is no Marxist romanticism for them. Emancipation comes through ideology critique. For Horkheimer, this means recognizing that traditional theory (e.g., science) becomes ideological when it fails to acknowledge that its knowledge is only one part of the interpretation of the world. Again, the George W. Bush administration is a good example of this, in that the clear attitude is that there is one correct way to see the world, and no others count. Any negotiation is compromise, and a violation of the modernist principle of reason.
Diverse forms of social life have to be manifest. This is ideology critique. Internal contradictions must be shown.