No offence, but you're 23...surely you're emotionally mature to know how to approach your parents about it.

"Mum, what's wrong?" Is usually a good place to start. If she tries to cover it from you, just tell her you're old enough to know what's going on.

You have to keep in mind that marital problems are getting more common nowadays. Alot of people (mostly younger than you) deal with it.