https://www.thedailybeast.com/princeton-student-edward-tian-built-gptzero-to-detect-ai-written-essays

GPTZero uses two different metrics to assess whether or not a text has been penned by a bot: perplexity, and burstiness. If the number is low, the likelihood of it being created by a bot is higher.

Perplexity is a measurement of randomness in a sentence. If a sentence is constructed or uses words in a way that surprises the app, then it will score higher in perplexity.

Burstiness is the quality of overall randomness for all the sentences in a text. For example, human writing tends to have sentences that vary in complexity. Some are simple. Some can give James Joyce a run for his money. Bots, on the other hand, tend to generate sentences that are relatively low in complexity, throughout the entire text.