Language proficiency for any language is usually measured separately by skill area: listening, reading, speaking, writing. Assessment of proficiency in listening & reading is based on one's score on a standardized test. For speaking and writing proficiency, a set of criteria is applied to an oral interview by an examiner, or for writing, to one's answers to two essay style questions. For most professions, the ability to write one of these languages is not a very important skill and so the exam typically does not include a writing portion.
Problems with sE Asian assessments
There are a number of problems with assessment of Southeast Asian languages currently. The first is the lack of available tests. A handful of universities (mostly outside the region), foreign governments and defense ministries/departments offer tests, usually reserved for their students, government employees, or military personnel. Tests for most of the common national languages stop at an intermediate level of proficiency and you won't find tests for the less common languages of the region at all. I have personally written to ASEAN asking them to develop a professional, official test like Japan did with the JLPT for each of the national languages of ASEAN countries, plus others which are important or widely spoken such as Javanese, Sundanese, Balinese, Cebuano, Ilocano, Shan, etc. Additionally, I have suggested the tests follow these guidelines: 1) Instructions are in very easy-to-follow English; 2) they follow an existing recognized scale (preferably CEFR), and 3) they are uniform in length and design. Whether ASEAN chooses to develop a set of standardized proficiency tests sometime in the future remains to be seen.
general problems with language assessment
Besides availability, there are problems common to language assessment in general. For any skill area, how well one does on a test depends to some degree on what the topics are about and the test taker's background. For example, when I was living in Korea at age 21 and working as a Korean Linguist for the military, I spent all my free time in bars or at home off-base speaking Korean. That, together with my specialization in certain military and technical aspects of the language, meant that I could listen to a fight between 2 bar girls or an artillery live fire exercise and translate every word into English as it was happening. On the other hand, a typical passage from a news story was completely outside my world of expertise and I would be lucky to understand half of it. A Korean drama about a love triangle and betrayal? - No problem. A simple ad in a newspaper? - No way. This type of specialization occurs more frequently than people realize. People can become highly proficient at a specialized vocabulary, while being barely adequate in the general language. Young people who hang out with their foreign friends become great at the everyday spoken lingo, but they can't write an academic essay to save their lives. And there are plenty of professional jobs and tasks that require specialized vocabulary, which is not tested on language tests. Think of the average American - you wouldn't hire him/her for medical transcription or to teach a course on IT, mechanics, or civil engineering, and yet these skills are required in the real world. As an EFL teacher who has had to learn a lot of specialized vocabulary and knowledge to teach English to math teachers, airline and cruise ship employees, industrial workers, and the like, I appreciate this fact. However, tests for specialized skills in SE Asian languages only exist to the extent a particular industry has developed one for its own employees (extremely rare, although ALTA Language Services might offer one or two).
A test is only as good as its design. At a minimum, the test has to be comprehensive enough to assess a general proficiency level on a fair range of common topics frequently found in the real world. This means the longer, the better. However, there are practical aspects that trump having a very long exam; namely, ease of grading (multiple choice), and comfort (the iBT TOEFL takes 4.5 hours and feels way too long). The other aspect that is traditionally difficult to incorporate into a paper-based test, is progressing level of difficulty so that someone at both a very beginning and intermediate level can take the same test and receive an accurate level assessment. This problem was solved a long time ago with computer adaptive testing (CAT) such as that used with the GRE. A good adaptive exam increases or decreases the difficulty of the next question based on the test taker's correct/incorrect responses and does an excellent job of determining a correct proficiency level. Unfortunately, these are still the exception for SE Asian language proficiency tests. Finally, a proficiency test should not allow wild guessing to positively affect the outcome (increase one's level). For a multiple choice test, this means that wrong answers should be penalized, but not weighted heavily enough to keep someone from going ahead and marking an answer they are somewhat certain about. In other words, it needs to encourage test-takers who understand most of a section and have a fair idea to guess, but to leave answers blank when they reach a point where they do not understand enough of the meaning to have a good chance of being correct. Unfortunately, this type of weighted test is rare for proficiency testing in these, or most, foreign languages.
There are several well-known scales for proficiency in a foreign language other than English. These include the Interagency Language Roundtable (ILR) used in America, and the Common European Framework of Reference for Languages (CEFR) used in Europe. Finally, one used extensively in Australia is the International Second Language Proficiency Ratings (ISLPR), which is similar to the ILR scale. The CEFR is only applied to a limited number of world languages. The ILR scale, on the other hand, is used by the US State Department, the US Dept. of Defense for its Defense Language Proficiency Test (DLPT) as well as by ALTA Language Services, which offers general and specialized proficiency tests in over 90 languages. This makes it the most-used scale for languages around the world, including all the national languages of SE Asia, although not many of less-common ones. Personally, I feel the CEFR scale has a few features which make it better than the ILR. However, I encourage you to judge for yourself; for a side-by-side look at the ILR and CEFR, read my comparison here.