You can use ya sabes when you know that your listener shares your understanding of a situation. It is especially useful for shared misery.
- Es Rodrigo, ya sabes. Well, it’s Rodrigo. And you know how stubborn he is. (The stubborness part goes unsaid. Its relevance derives the context and your shared history with Rodrigo.)
- Ya sabes cómo es. You know how she is. (She’s so fussy. She won’t want to eat at a place like that. So let’s just rule that option out.)
To force someone to spell things out, you could answer Ya sabes with ¿Ya sé qué? (said with falling intonation) What exactly is it that I know what, according to you?
The tag ¿Sabes? works for You know that, right? For example: Siempre voy a estar contigo, ¿sabes? We’ll always be together, you know that, don’t you?
Standing alone, ¿Sabes? is also a shorter and more informal version of ¿Sabes qué? It is useful for bringing up something you just remembered. ¿Sabes? Olvidé tu número de teléfono. You know what? I forget your number. Could you tell it to me again?
Besides meaning <>Guess what?, ¿Sabes qué? can preface an angry retort: ¿Sabes qué? Ahórrate tus explicaciones. You know what? Just save your breath.