What is the solutions for a binding to know if an item is already loaded (ready for writing) ?
The question is relative to openHAB startup process.
Should I use itemRegistry.geItem to check if item is already crested ?
No, you should not use the ItemRegistry.
The binding should (and afaik all of them do) implement BindingChangeListener - if they get a call through this interface, they are either directly informed about an item (bindingChanged) or can ask the BindingProvider (in allBindingsChanged), if it has a configuration for a certain item - if not, you can assume that it has been removed (or is at least non of your business).