" 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 !