You slip in random words in the other language, without even noticing. Then you will either be met by concerned stares or be called out for it (mostly by friends who wanna tease you)
You will forget if you read a book in your first or secondary language. The same goes for Tv-shows, movies, conversations you have. Sometimes you won’t even realize you are consuming something in your secondary language, until someone points it out, or you’re trying to enact that funny dialogue in that one episode.
You will have to look up a word and feel like the ultimate failure. Even though that happens in your first language too, since, you know, sometimes a word just doesn’t come to mind. You will feel worse if you didn’t know the word to begin with. You will make mistakes and feel horrible about them, even though they’re mistakes you’d simply brush off in your first language.
You will breathe a sigh of relief when you are offered to do a presentation or assignment in any language you prefer, because first of all, you know that everyone will be so impressed by your skills that they won’t notice you actually know nothing about the topic at hand and secondly, it honestly comes easier to you. The same applies for required reading or writing.
People will ask for your help with the language all the time. You will feel oddly proud.
A lot of the time, you will probably dream in your second language. You will start to wonder if something is wrong with you. It isn’t.
People will start wondering about the origin of your skills. If it’s english, chances are, you simply spend a lot of time on the internet.