The Japanese-Language Proficiency Test, or JLPT, is a standardized test to evaluate and certify Japanese language proficiency for non-native speakers.
There are 5 levels: N5 being the lowest level up to N1 which even native Japanese struggle to pass. Each level gets exponentially challenging.
For example, all the characters you see here are the kanji (漢字) you need to master to pass N5. N4 has another 500 kanji, N3 has another 1500, and so on. This is only kanji, we still have listening, reading and grammar to tackle.
To work or study tertiary education in Japan, one needs an N2 certificate.
A survey by the Japan Foundation reveals that 33% of applicants took the test just to measure their own level of proficiency.
In short, taking the JLPT is the only way to know if your Japanese is any good.