There have been many suggestions to trial a tip warning device with beepers etc but most are impractical and would most likely prove an unwanted distraction in a busy environment.
The winch operators are the key and they have to constantly scan tip and tail clearances whilst trying to watch the winchman's hand signals and voice marshal the aircraft.
On many helicopters the pilot sits quite far forward relative to the rotor hub so he would have to look over his shoulder to see the widest part of the rotor disc (not very comfortable or practical for accurate hovering).
A good training exercise on the ground is to have a member of the crew or ground crew stand just outside the rotor disc so the crew in the helicopter get used to judging the distance.
Our winchmen have Polycon (a winchmans intercom/radio) when they go on the wire but more often than not their attention is focussed on facing forward and getting safely to the casualty.