Well the fact that companies' valuations are disconnected from their fundamental analysis would be the definition of a bubble. I expect that active investors will bet against hyperinflated companies beforehand, so I think that this is more a theoretical issue. However, I would definitely scrutinise the companies that are barely included in the index (and those that are just excluded) because these are the companies that have more to lose or to gain from being included\excluded in the purchase list of an index fund.

These would be the best target for a short position in my opinion, if we believe their valuation are artificially inflated.