AI Text Classifier is a free AI content detector by OpenAI, the creators behind ChatGPT and other GPT models. The tool is still in development but you can already get access to the work-in-progress build. There are a few limitations, such as a 1,000 character minimum limit. You need to provide the AI enough text to work with, OpenAI also mentions that the accuracy of the tool is not guaranteed, although it is constantly improving.
Update: Sadly, it looks as though OpenAI has discontinued the AI Text Classifier tool due to low accuracy. You can find some great alternatives in our AI Content Detectors category!