Your question is Seasonality Estimation. Take a moment with it on the right.
Talk me through your thinking if you like. When you're confident, submit your answer and I'll grade it like a real screen (7/10 or better passes).
How would you estimate seasonality, in the simplest way possible?
Keep the answer practical and brief. Describe the simplest method you would use, what data you need, and how you would check whether the pattern is real versus noise. If helpful, mention the minimum aggregation level and one limitation of the approach.