I'm an English Language Assessor and have done stacks of formal assessments over the past four years.
In the UK Flight / RT Examiners can do INFORMAL Assessments but as previously stated can only assess a candidate at Level 6 and then only if the candidate is fluent and meets the criteria as defined in the ICAO English Language Descriptors. (http://www.aviation-esl.com/scale.doc.pdf
- PS. I do not work for this company)
If the candidate does not meet this criteria the Flight / RT Examiner cannot sign him/her off at Level 6 and he/she must be formally assessed by a person trained in English Language Assessment.
The problem is that, unfortunately, there are a number of Flight / RT examiners who tick the Level 6 box, regardless of the candidate's ability and I have personally seen some worrying examples whereby people who do not even meet Level 4 criteria have been signed off at Level 6. Therefore, the Flight Examiners concerned have authorised those persons to fly internationally when those persons do not have the ability to communicate adequately in English.
Regrettably, the names of certain ATO's crop up time and time again but I am not prepared to name them here.
In defence of the Examiners, they have not received training in this area and do not know what to look for. As is very obvious to a trained assessor, just because you can understand someone does not mean they automatically meet Level 6 criteria. Furthermore, most of the examiners I have talked to have no idea what to do if the person is not fluent in English and cannot advise the candidate where to go for formal testing so the easiest thing to do (and human nature kicks in here) is sign them off at Level 6 and trust (hope?) that nothing goes pear shaped in the years to come.
This aside, there are many airlines that insist on proven certification from a formal assessment, therefore, the informal Level 6 sign-off is not worth the paper it's written on.
I have had to do many assessments for native-English speaking airline pilots now working overseas where the airline has required formal certification and I have even assessed an English pilot for a British Airways job.
Also, at the moment nothing is printed on the UK issued licences to state the assessed level of English - all it states is "Language Proficiency: English" (or something to that effect) but no level.
My advice is that, if you intend working overseas (native English speaker or not) get an assessment done formally as you will have the certificate in your CV portfolio and it saves any headaches at the last moment.
There are plenty of places around that do assessments, apart from my own organisation, so you should be able to find one in your neck of the woods.
Under EU Regulations, Level 4 (Operational) needs retesting after 4 years, Level 5 (Extended) after 6 years and Level 6 (Expert) is valid for life.
If you have any further questions, please feel free to drop me a PM.