Mainly people see it as binary (Pass/fail) when it exists on a spectrum.
The Two main metrics are:
1) How many nodes are on the network.
2) Difficulty to attack the network.

1 gives you how many copies of the ledger exist out in the world. (Censorship resistant from the past to the present)
2 gives you censorship resistance in the present and near future.


With this definition.  If two networks (Bitcoin and Ethereum for example) have equal  amount of nodes on the network and cost to attack. They are similarly decentralized.

There's other parts of the equation that play a role in how theoretically decentralized a network can be. Like hardware requirements. But that's secondary to its current state.