afab people typically type in lower case and use more abbreviations/acronyms and more likely will keysmash, use certain memes or use "!!!" or ",,," or lots of dots or all caps. also some specific phrases just scream afab to me. amab people usually are more blunt with typing and use capitalization (not always) and grammar more often, most likely won't use any !!!s, may keysmash if it's a gay man but unusual, more dry sense of humor, might be more likely to use longer words and reference politics

you are insane