How do i know my Shih-Tzu is 100% Shih-Tzu?

Ok well i just got this 10month old female Shih-Tzu. She is all black, long ears down aside her face. Short stubby nose, long somewhat squared body, she looks very built. Her tail comes around onto her body kinda curved. Short stubby legs. I have googled several images and she matches the critiria, but my neighbor just looked at her and said she is mixed with weenie??? I doubt it though i think he's just mad cause i got a better dog then he has.