A convergence in prices doesn't necessarily represent a cartel-it can just as easily represent fierce competition whereby all the factors have beaten their prices down to the bare minimum.

Over time it's easy to tell the difference. In a cartel the prices are artficially high and so the final price is governed by what customers are willing to pay and so it doesn't fluctuate. Conversely a fiercely compettitive market has tiny margins and is extremely sensitive to supplier costs so it does fluctuate.