How can I tell whether a regulator protein is an activator or an inhibitor?

A regulator protein that activates transcription by helping RNA polymerase bind to the promoter is called an activator. A regulator protein that inhibits transcription by making it difficult for RNA polymerase to bind to the promoter or start transcription is called an inhibitor. You can tell whether a regulator protein is an activator or an inhibitor by studying what happens when it binds DNA. If it turns genes on when it binds DNA, it is an activator. If it turns genes off when it binds DNA, it is an inhibitor.