If you're near someone who burps:

Simply smile a little smile that says, “Been there, done that!” Do this well and you're going to be a hero! If you feel you must say something (really, though, you don't have to say anything), a simple, “Of course” or ”Sure” is all that's needed.

How do you burp discreetly?

You can be discreet by muffling a belch or holding it in for a few seconds until you're in a more private setting.
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Otherwise, it could be caused by any of the following:

In what country is it polite to fart after a meal?

4- Is it ever polite to fart after a meal? Articles have been circulating the web that the Inuit people of Canada fart after a meal to express thanks and appreciation after a meal. As interesting as this sounds, I found no proof of this whatsoever.