For indicating rotational displacement between an Allen bolt having a bolt head (34) provided with a central recess (36) for engagement by a fastening/unfastening tool, namely an Allen key, and a substrate to which the bolt is fastened, thereby to indicate whether the bolt has loosened, a safety indicating device is provided comprising a body (10) having a bore formed with splines (12) for engagement over the bolt head (34). The body (10) also has an end closure (14) formed with a plug (16) projecting inwardly of the bore for snug engagement into the central recess (36) of the bolt head. An indicator is also provided on the body (10). This is typically a pointer (18) mounted or formed on the body and projecting laterally, but it could be an indicator which is merely marked or formed on the body without projecting outwards. The plug (16) may itself be formed with the recess (26) which may be used for insertion of a different tool to effect rotational adjustment of the position of the device upon the bolt head (34).