I want to test a hypothesis about using gamma to predict FX movements.
Suppose that market makers will seek to be delta neutral given their portfolio of FX options. At any given time, market makers in the interbank market will be either long or short gamma in the aggregate for their portfolios of FX options. If they are long gamma, market makers as a whole will delta hedge by engaging in strategies such as continuously buying the spot when it is low and selling it when it rises, meaning the spot range will operate in a relatively tight band. The opposite will hold if market makers are net short gamma, with the spot swinging widely.
If I have access only to public information (eg anything on Bloomberg Terminal) how can I identify whether market makers are long or short gamma for FX options in order to test this hypothesis?