This paper gives an information theoretic approach for detecting Byzantine modifications in networks employing random linear network coding. Each exogenous source packet is augmented with a flexible number of hash symbols that are obtained as a polynomial function of the data symbols. This approach depends only on the adversary not knowing the random code coefficients of all other packets received by the sink nodes when designing its adversarial packets. We show how the detection probability varies with the overhead (ratio of hash to data symbols), coding field size and the amount of information unknown to the adversary about the random code that reaches a sink node. This approach can be used in conjunction with higher overhead schemes that are activated only upon detection of a Byzantine node.