How do I know if I'm in too high a gear uphill?

[PenguinLust]

I like to save gas and drive in top gear as much as I can. It's never been explained to me in any detail when I should gear down going up a hill. The expressed reason for this is to achieve/maintain the power you want, but I understand it can also damage the engine if you don't. I've tried Googling things like 'car gear uphill', and it seems that "lugging" is one of the danger signs. Does that mean if my car is not lugging my engine isn't taking on any damage? And how do I know if it's lugging?
I tend to drive my 2006 Civic at 80-85km/hr (~50mph).