A system and method for determining the modulation type and constellation order of an input signal given that the modulation type and constellation order are known to belong to set of possible types/orders. A computer operates on samples of the input signal to compute signal features. The computer classifies the modulation type and the constellation order by comparing each feature to one or more corresponding threshold values. The results of the comparisons are used to determine the modulation type and constellation order of the input signal from the set of possible types/orders. The classification may be performed based on a decision tree, each leaf node of the decision tree corresponding to a type/order combination, and each non-leaf node corresponding to one or more of the comparisons. The set of possible types/order may include PSK with a number of different constellation orders and QAM with a number of different constellation orders.