Use Compare alt coins to the "altcoin index", an average of a selection of alt coins. Above zero means the coin you are viewing is worth more than the index, indicating the possibility of overbought. Below zero indicating the opposite, a possibility of oversold. Press the cog icon to select which alt coins you want included in the index.
Notes The coins in the index are ALTBTC, meaning their value against BTC. Due to this the indicator is useless when viewing BTC only charts.