There is more then one definition regarding centralization, but the most common is that you need to have more then half (51%) of the mining power.
A fork is something that has nothing to do with centralization as such. Everybody can fork at anytime, but if you fork and nobody uses your fork, then you forked off to a worthless (alt)coin.
Bitcoin has a problem if we fork into two more or less evenly big coins. Such a fork can be initiated by anyone. You can fork with the goal to be more decentralized after the fork, but if and how you achieve this is a different matter.
This leaves only the question how to make sure that a fork has a clean code. Firstly you need the code to be open source and then you can look at the code and see for yourself if it is ok. If you are not able to do so then you will have to trust other who will check it and say if it is ok. But this is the same thing you do now with Bitcoin since you probably don't understand the code we use know. The difference is that many more people have check the Bitcoin code then any other fork till now or in the future.