There's no definite way to tell that a dip ends or not, but there are a lot of resources that's going to help you which are already mentioned by other users. Technical analysis I think is a good way to tell but not accurately predict if it's the bottom or not, try to also read a lot of articles regarding bitcoin and cryptocurrencies because those news can affect the prices.
Dip is not end at anypoint. Only thing is, the price will dip and pump. It occur in simultaneously. If you check the price of any coin over 1 year, you may find this one. But repeated dump and pump will happened with certain interval. It won't occur with short period. You need to patience to get such things happened.