Perimenopause is a process — a gradual transition. No one test or sign is enough to determine if you've entered perimenopause. Your doctor takes many things into consideration, including your age, menstrual history, and what symptoms or body changes you're experiencing.
Some doctors may order tests to check your hormone levels. But other than checking thyroid function, which can affect hormone levels, hormone testing is rarely necessary or useful to evaluate perimenopause.