IsPlayerOnAnyVehicle(playerid)
{
  new Float:vehx, Float:vehy, Float:vehz, Float:plyx, Float:plyy, Float:plyz;
  GetVehiclePos(v, plyx, plyy, plyz);
  for (new v; v < MAX_VEHICLES; v++)
  {
    if (GetVehicleModel(v) >= 400) // not sure about this line
    {
      GetVehiclePos(v, vehx, vehy, vehz);
      if (PlayerToPoint(10, playerid, vehx, vehy, vehz) && vehz + 2 < plz)
      {
        return 1; // could also return the vehicle ID (v)
      }
    }
  }
  return 0;
}