bool:IsSurfaceSliding(client)
{
      decl Float:start[3], Float:normal[3], Float:end[3];
      GetClientAbsOrigin(client, start);
      TR_TraceRayFilter(start, Float:{-90.0, 0.0, 0.0}, MASK_SOLID, RayType_Infinite, TraceEntityFilterPlayer, client);
      if (TR_DidHit(INVALID_HANDLE))
      {
       TR_GetEndPosition(end, INVALID_HANDLE);
       TR_GetPlaneNormal(INVALID_HANDLE, normal);
       GetVectorAngles(normal, normal);
       if (normal[2] >= 30.0)
        return true;
      }
      return false;
}

public bool:TraceEntityFilterPlayer(entity, mask, any:client)
{
         return client != entity && (GetEntPropEnt(client, Prop_Data, "m_hOwnerEntity") == mask);
}