An internationally accepted indicator is the current account balance to GDP ratio. For example, the current account norm is the core concept in the macroeconomic balance (MB) approach, which is used by the IMF to assess member country’s exchange rate.
If a country’s current account balance exceeds the norm in the medium term, its exchange rate is believed to be undervalued, and otherwise the exchange rate is overvalued. The size of the gap between current account balance and the current account norm indicates the degree of undervaluation or overvaluation.