Except sometimes people of the same sex will underrate or act like someones not attractive out of jealousy. The opposite sex is definitely the most accurate way to determine how attractive you are as unless someone of the same sex is gay than they won't be attracted by you so their opinion isn't as valuable. However I do think that someone who's considered to be good looking by the opposite gender will likely be recognized as good looking by the same gender as well, even if there may be some sugarcoating the truth due to spite or jealousy.