Which expression is more polite? Could you or can you?
If I can't find something in a grocery, which question is more polite? "Could you tell me where I can find the mouthwash?" or "Can you tell me where I can find the mouthwash?" And I'm not sure "Could you tell me ...." is a common expression for asking or needing help.

Question

Which expression is more polite? Could you or can you?
If I can't find something in a grocery, which question is more polite? "Could you tell me where I can find the mouthwash?" or "Can you tell me where I can find the mouthwash?"  And I'm not sure "Could you tell me ...." is a common expression for asking or needing help.

Answer

Can you...?
Could you...?
Would you be able to...?

The above 3 are the most common polite phrases to request something, from weakest to strongest.
You need to consider how big the request is and your relationship to the person.
So in a shop, "can you...?" is perfectly polite because it is their job to help.

Answer

For being really polite you can say “Could you please tell me where I can find the mouthwash?”. 
Yes “could you tell me...” sounds really natural, and it’s pretty common.

I hope it helps, :)

Answer

I agree with Luke and Hasmik, "could" is more polite.

Answer

To be very polite, I would use, "Could you please tell me where I could find the mouthwash?"

Typically, I would opt for, "Where is your mouthwash located?"