I would recommend taking an exam through the Goethe Institute. It's the premier organization for German as a foreign language and they have offices/offer testing all over the world. You can take any level of German (using the standard European system of A1, A2, B1, etc.). It's not only generally accepted as proof of proficiency, but they're definitely not just for advanced speakers! They also offer classes specifically tailored to their own tests, but you can of course take the test without the class.