In this paper, we investigate symbol-level selective decode-forward (SDF) relaying for uncoordinated dense wireless networks. The main objective is to establish reliable communications for such wireless networks, which cannot have accurate link budget allocation due to signalling overhead limitation. To approach this goal, a novel half-duplex absolute difference based symbol-level SDF is developed to enjoy diversity gain at destination node with no signalling between relay and destination node. The proposed selection criteria is designed that no preset threshold is needed at relay, i.e., receive SNR threshold for uncoded systems or LLR threshold for coded systems. Therefore, it is more suitable for the considered uncoordinated dense wireless networks. It offers better performance, in terms of symbol error rate (SER), when compared with the conventional always decode-forward (DF) relaying and frame-level SDF. Computer simulations are carried out to evaluate the proposed approach with respect to the frame length and channel gain. Significant performance improvement is observed in terms of both SER and FER with various setup of frame length and channel gain.