https://arxiv.org/abs/2202.03629
But you can get a general feel by using ChatGPT. Open up a new conversation and ask it something like, "What is the capital of France?". Note the response. Open up a new conversation and note the response. Soon enough you should be able to see that the responses are far from random.
You can use the OpenAI APIs directly and have it run 10,000 or so iterations to see what kind of "hallucinations" it makes! They are not random!