If the student has taken any steps to disguise that it was AI, you're never going to detect it. You best bet is having read enough awful, verbose generative text output to get a feel for the garbage it outputs when asked to write essays.
While most instructors are going to be focused on whether it can successfully detect AI-written content, the true danger is detecting AI-generated content where there isn't any. In short, AI detectors look for predictable text. This is a problem because boring students writing boring essays on boring topics write predictable text.
As the old saying goes, "it is better that 10 AI-generated essays go free than that 1 human-generated essay be convicted."