How can I determine my language level

Sometimes people who are writing me, say that they have reached a certain level in the language, for example B1. I always wonder how they are able to do that? Is there a reference: 10000 known words means level X, or can you only determine it when finishing a course book on a certain level?

Thanks

If you really would like to determine your language level, you habe to pay attention not only to a certain number of words, but also to your possibility of reading, writing, listening and speaking - to meet all these requierements.
All these specifications you can find in the Internet.
But in the real life if you are not a teacher or a translator it’s quite enough to feel comfortable in all these parts or only in the part of the language that is more important to you, for example -in reading and speaking or only in speaking.

ok thank you, I was just curious, I don’t think I need it, because I am only learning for fun!

Just do a test, and you know your level: http://www.sprachtest.de/

thx and danke for the links, I will have closer look there later on