I advise you to commit this important way of asking, “How much do I owe?” in Italian to memory.
By using “Quanto le devo?” you will be demonstrating that you’ve done some homework on the importance of respectful communication in Italian culture. And it is a phrase that you will employ daily when you step to the “cassa” (cash register) to settle your bill, particularly at bars and cafes.
I cringe when I hear non-Italians just saying, “Quanto?” (How much?)—and without a simple “Salve” (hello) first. “Quanto?” or “Quanto costa?” (How much does it cost?) both do the trick, but they won’t earn you friends nor endear you to Italians. This is where the American Way falls flat and is in stark contrast to respect as an integral part of life and communications in Italy.
Let’s deconstruct “Quanto le devo?”
Okay, we know what “Quanto?” means. “Le” is an indirect pronoun and an abbreviated form of “a Lei” (to you, singular formal). “Devo” is “I must” or “I need to.” You might be scratching your heads, thinking that when you put it all together, you have a clunky phrase that says, “How much must I do you?” No, it means, “How much do I owe you?” The important callout here is the form of “you” used is formal (respectful).
Showing respect by using “Quanto le devo?” is just one of many important ways of communicating with respect. I’ve climbed on my soapbox many times to caution my fellow Americans to take respectful communications seriously in Italy. And, if you’re learning Italian with a tutor, please, please, please, make sure that person schools you in the essential nature of respect in communicating in Italy. Sure, you’ll find loads of Italians speaking improperly or in slang (a lot of them have become caricatures in TV and movies), but don’t use them as a license to be lazy (and disrespectful).
Asking for the bill (while still seated) is a different matter.
Technically, you could use “Quanto le devo?” while still seated a restaurant. But “Il conto, per favore” (The bill/check, please) or “Posso avere il conto, per favore?” (May I have the bill, please?) are most appropriate.