How do you detect seasonality(multiplicative or additive) in a time series data?
Once divide the time-series by its Moving Average and once subtract the moving average from it.

If the seasonality is additive, then the result of subtract will have almost similar fluctuations in magnitude.

If it is multiplicative, then the division result has such a property.

Like the figures below from my course in Feature Engineering:





So the seasonality was multiplicative as the division has similar magnitudes of fluctuations.

Please note that more linear the dynamic of the time-series is, better this naive approach works.

Hope it helps!