You need to pay attention to what leads the growment of the *bubble*. In my opinion it's bitcoin which leads everything in this market. While bitcoin is doing well all these altcoins will keep fine, so just be attentive to bitcoin's price and news. Once bitcoin bull run ends, it's time to cashout your altcoins' profit, if you are a short run investor.
But if you want to invest on long run you shouldn't worry about bubbles.