In this letter, a detect-before-decode (DbD) method for relay misbehavior detection in a wireless decode-and-forward (DF) cooperative network is proposed. Since this method makes no use of the decoding results, the detection errors caused by erroneously decoding are eliminated. A unified sufficient statistic for relay misbehavior detection, which is applicable to all link outage conditions, is constructed. Furthermore, the closed-form expression of the optimal threshold which minimizes the source-to-destination outage probability is derived. Numerical simulations show that the proposed method outperforms the long-term relay link observing method in terms of false alarm and miss detection rates. It has the same false alarm and miss detection rates as the physical integrity check (PIC) method with lower computational complexity. The outage probability performance is also improved when transmit power is low.