I guess you would look for correlation in price changes. Problem with that is that external factors (oil prices, failed harvests, earthquakes in Japan) can cause correlated price changes, too. Correlation of price changes between some major competitors while some other players do not follow suit might be an indicator, but those other competitors might just be in a slightly different situation (other manufacturing method or other company structure (e.g. if one of them owns a mine or has a long-term contract, price changes would affect them differently))

In the end, it is really hard to detect cartels. That, I think, is the reason the EU has the rule "the one who tells us walks free". Without it, companies taking part in cartels would have too little to fear.