> When is the appropriate time to intervene to ask if everything is ok?

Where I grew up (The Netherlands) this simply does not exist. If you need something you ask the waiter and only then they'll come to you. Likewise for the check, the check only comes when you ask for it. It is considered extremely rude to get the check before you've asked for it.

Of course, after living in the US for 5 years I've become accustomed to all kinds of weird things (like the insistence of having salad before the entree, and entree not being an appetizer, simply blows my mind).