As Hizu said. Only trust certain people and if those certain people whisper you and claims to be them, ask them to log onto their main. If they're already on their main, ask them.
Usually the asking to log onto their main would stall them, sometimes they'll come up with bullshit excuses such as 'got banned'. If your friends seriously got banned, they'd let you know somewhere else. :/
Also, friends would also know your quirks. For example, most of my friends know that I hate it when people log onto characters I'm not familiar with and say 'guess who?
'. I know too many people, what makes you think I'd want to figure out who you are? You could be a poser/scammer too for all I know. Hell, you and I may have never talked before. If it was someone I don't/barely talk to, I'd be suspicious right off the bat.