Here in Britain classification is by a suitably qualified examiner. Level 4 is a minimum standard, 5 is high proficiency but not native, 6 is native standard. It's pretty obvious to a native English speaking examiner if their candidate is at level 5 or 6. 4 is more ambiguous, but can be awarded and tends to be to minimum standard candidates from non English speaking countries.
There are plenty of online services for training and testing - just search online. I would suggest looking to Europe, not the USA - Americans speak a dialect of English that can be somewhat different to ICAO standards. In Europe people like the French or Germans will be working in a second language, so tend to be much more careful about their adherence to ICAO language standards.
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