  A novel criterion to determine the presence of gravitational radiation
arriving to, or departing from, null infinity of any weakly
asymptotically-simple space-time with vanishing cosmological constant is given.
The quantities involved are geometric, of tidal nature, with good gauge
behaviour and univocally defined at null infinity. The relationship with the
classical characterisation using the News tensor is analysed in detail. A new
balance law at infinity is presented, which may be useful to define `radiation
states'.
