The Common European Framework of Reference for Languages (CEFR) provides a user-friendly tool for assessing your own level in a particular language, based on what you can actually do in that language. The CEFR self-assessment grid allows you to evaluate your skills in the areas of listening, reading, speaking and writing, based on a system of 6 levels, from A1 to C2.
To assess your level of English, you can use the grid in any language, e.g.:
The grid is also available in 29 other languages.
If you want to have your own assessment confirmed by doing a "proper" test, you might want to try one of these: