We recently had a game were a PL settled on that hero at the last moment, and our team has no heroes with AOE abilities. Pick strategies aside, what are some items we could have gotten to do AOE damage and identify the illusions?
There are only four AOE damage items which are:
Battle fury (cleave)
Mjollinir (lightning jumps)
Shiva's Guard active (ice and slow)
Realistically your best bet is with battlefury as that is based of hero damage rather than a damage over time or proc chance of lightning, so will show a bigger difference of health loss between the real PL and illusions.
Normally, however, by the time PL gets fully up and running you need to have heroes that can counter him (OD, sven etc) rather than relying on items.
What you and franglais have said is fairly accurate, you want to get something that has cleave or will hit multiple targets at once: Battlefury, Mjollnir, or Radiance. If you already have a few heroes who can do some magical aoe damage, getting a Veil of Discord is not a terrible idea, as it amplifies magic damage in an aoe. If you want survivability against a PL, you could maybe go for something like a Ghost Scepter to stay alive for just a little bit longer. Gem is an option too, simply for knowing which one is the real PL, but then you take the chance of losing it when he figures out who is holding it.
I feel it's necessary to really stress that if you have nothing to fight PL with aside from items, you lost when you picked. When an enemy player notices you have no aoe and knows that PL is great against lineups with few abilities that are good against him, he's going to pick PL. It may just be matchmaking but there still is a degree of strategy that should go into picking. It's like if a team were to pick Brood, PL, and Naga, then the other team pick Sand King and Earthshaker at the last moment. Sometimes there is not much you can do to counter a counter.
As an example, mjollnir seems to be a frequently recommended item against PL, for its passive of lightning charge. It affects upto 7 targets, but is blocked by magic immunity (which may not be a bad thing - would it lead to an even greater and easily identifiable difference between the hero and his illusions? ).