Duolingo provides no specified syllabus for the DET exam as it is an English Language Proficiency Exam. However, you are advised to prepare and practice word spelling, listening conversations, read & understand the meaning of the sentences and write & speak about random topics. You must also be an analytical thinker, write about the scene of an image, and think about social issues.
Duolingo English Test comprises 3 sections: the Quick Setup, Adaptive Test, and Writing & Speaking sample.