Enumerated qualifier values can be looked up using "QualifierValues":
In[1]:= EntityProperty["Country", "ExternalBalance"]["QualifierValues"]
Out[1]= {"CurrencyUnit" -> {"Constant2000USDollar",
"Constant2005InternationalDollar", "ConstantLCU", "CurrentLCU",
"CurrentUSDollar"}, "Date" -> {},
"PercentOfGDP" -> {"PercentOfGDP"},
"TradeSection" -> {"GoodsAndServices"}}