As an ASL Interpreter/Teacher I think it's important to have an assessment done periodically however I also feel that the best way to learn any language is total immersion or at the very lease through socializing with native users of the language, whatever language that may be. The thing with a language is it's almost like a living thing meaning it grows and changes with the times. New words are added to it as are new rules at times. The biggest thing with getting assessments rather it be a professional one or through native users is it helps you to not only know if your doing it correctly, but it also helps you to find out just how much you actually understand when either hearing it or seeing it. I know with ASL receptive (seeing and understanding) skills are generally, not always, but generally the last thing to develop and the first thing to lose. it takes a lot of practice and as I always encourage my students the best thing they could do is go to a place in our case a Deaf social where there are lots of opportunities to use the language in question and get feedback.
\Also keep in mind that culture plays a huge part in learning a language, understanding the culture as well as the grammar syntax to the target language Even after 41 years and knowing this language all my life, I still will tell my Deaf and Hard of hearing or hh friends to let me know if I do something wrong regardless of how small it is because I know that that is the only way to learn. For all I know there may be a new way of signing something or a new term/slang to learn. In the line of work I do Interpreting as well as teaching, it's good to know all those things so if I mess up. I want them to tell me.