Use academic English when talking to teachers, in a business setting, or when writing an essay. Use slang when talking to friends. You can use academic English when talking to friends, but if you talk too formally you might sound weird. If you google a word and an urban dictionary definition shows up, it's slang. Other than that, you will just have to learn the difference. And you can always ask online here, of course.

I think you can answer the first two questions yourself. ;) "Is it fine if I use academic English when I talk or chat with friends?" It depends on the friend. Many of my friends are conscientious and well-spoken, so we take care with what we say. Other friends have slangy habits, which is OK if I'm just chatting to them. If you use academic English with someone who wants a more relaxed style, then you'll seem cold and even arrogant. Conversely, if the person is more careful with their language, your slangy style will look a little uneducated and rude. Pay attention, and you'll learn to adapt to each situation. If you're not sure, stick to standard English. Nothing wrong with that. "Can I use general language when I write an essay or do academic research?" If mean colloquial and slang forms, then no. The reader wouldn't take you seriously. "How can I know whether a word is academic or colloquial?" I do a two-hour workshop on the difference between formal and casual English. You'd be surprised how easy it is to tell, though I'm not sure how I could condense it into a couple of lines.

thanks so much guys

I always use academic English. I taught academic English to my children. It is the language I speak at all times. I teach English to speakers of other languages and they respond in academic English.