The answer to your question depends on how precise you need your results to be. There are several free tests online, as evidenced by a google search I did of "english level" (without quotes). Some links I found were:
English Level Test Online,Free Tests of My Language Skills
English Grammar Placement Test
English Language Test
If you average these results, you'll probably find slightly accurate data.
If you want your test results to be more accurate, you should probably sign up for an official test. One link for information about more official tests is English Level Test - Which English exam is right for you?.
Hope this helps!! :)