The formal proof would be University transcripts in English, let's say from India, from a University. Since the transcripts/ degree certificate
does not explicitly say that the instruction was in English, you include a letter from the University (I don't consider this is a formal),
so if you also included a letter from an employer, let's say, you are now a professor teaching students in English here in Canada (so a letter from your Department Head). Don't you think this letter now would carry more weight than a letter from an arcane University back in India which the CIC officers have never heard of?!!...
Again, the third party score is easily obtained and is the easiest and most uncomplicated way...