What's cryptocurrency?
It's a decentralized encrypted exchange unit.
My first time to know this definition about cryptocurrency. Also not all cryptocurrencies are decentralized.
It's transactions are visible through a decentralized ledger (Blockchain)
Transactions can be visible, traceable with any public ledger. You don't need to have a decentralized ledger to see transactions and trace them. A public ledger can help you to do so and it can be either centralized or decentralized.
How can you tell which cryptocurrency will go up ?
Use case and utility of a cryptocurrency will made up its value. Without good use cases, good utilities, it's hard for that cryptocurrency to gain value.