" Tu sabes bailar" could be both , a statement or a question . If you are confirming that someone can dance , as george says , then it is a statement ,you are just telling your friend that he can dance " tu sabes bailar" , yo creo ,que sabes bailar " i think you can dance " for example If you are wondering and questioning if someone can dance or not then it is a question and it takes and (?) interrogation mark at the end , if in the written form ; " Tu sabes bailar ?" If it is oral an just said , you will certainly know from the tone of speech if it is a question or a statement !

If you are tryng to confirm that the person know how to dance.. then is right to say TU SABES BAILAR. I

I think it would be a question because you are asking them if they know "how" to dance. Therefore, it would be a question. I may be wrong.