Hayami Kamachi

There is really no way for you to tell for sure. The best you can do is ask them what their sex is in real life, and if they give you an answer, in good faith choose to believe they're telling the truth. Even if it does happen to be a man playing a woman avie (or vise versa), it doesn't reflect negatively on you, because there is really no way you could have known in the first place.