I want to improve my English, especially writing and speaking. Therefore, I need an indicator of my current level of English competence. Having a reliable metric in hand, I would be able to see how my English skills improved with time.
Is there an official, internationally acknowledged metric for English language competency?
Can someone provide a reliable resource to such as assessment tool?
You can assess yourself against the criteria used by the European Union here.
First off, it's important to understand that the measurement consists of several metrics at once. Having a single metric is simply useless, to my mind.
There are several internationally acknowledged testing systems. As far as I know, all of them contain three major assessment steps:
(paywall: tests and official training materials are not free)
Being accepted by most academic institutions in Australia, Canada, New Zealand, and a large majority of those in the U.S., this system seems to be one of the most interesting.
There are four "Modules" — Listening, Reading, Writing and Speaking — for each of those a student obtains a band score (9 = "Excellent User", 8 = "Very Good User", etc).
Universities require a certain level of English comprehension, measured in IELTS units. Usually it's between 6.5 to 7.0, but some require up to 8.5.
They provide with Practice Materials, and all books I've seen contain self-assessment modules.
When you have done, visit one of their 800 test centers worldwide to graduate.
(paywall: tests and official training materials are not free)
The Modules are essentially the same as for IELTS, but scoring differs a bit.
As per training materials, you may google for "Barron TOEFL", "Cambridge Preparation to the TOEFL", and many others.
There's also a TOEFL blog where some interesting links appear.
Self-assessment online:
Barrie England's answer is correct, the CEFR is a very helpful guideline but if the OP is interested in taking exams to measure his level and prove to others that his English is progressing I would say that: TOEFL is awful but IELTS is better.
As I understand it, although I'm not an expert; TOEFL has one fixed format (which by the way is continually updated) and is best taken by students wishing to enter the American university system. The computer based exam is very challenging and demanding, but it is the listening and speaking sections which verge on the sadistic that convince me not to recommend TOEFL to learners. For example, in the speaking part, the candidate speaks, alone, directly to the computer microphone. His/her answers are digitally recorded and then assessed by an external examiner. There is no interaction. The listenings are not repeated, and at the same time you are expected to take down notes which will later serve to write one or more essays.
For those who want to see how their level of English compares with that of native speakers; a fairer, and more humane assessment would be the Cambridge ESOL exams. Look under the "General English and for schools" tab. Exams are divided according to the CEFR guidelines and are therefore easier for learners to decide which exam to sit. The information on the website is also exhaustive about the different formats available etc.
An alternative examining body is Trinity. Unfortunately, I have no direct experience of the exams but I am aware of their growing popularity. I do know however, that the speaking tests are performed face to face and appear to mimic a more realistic and natural situation. http://www.trinitycollege.co.uk/site/?id=1803
