This is kinda tough, even though it's common. One way is to consider their oldest child's age, if they have any children. If he/she is older than or the same age as you, so yeah, I guess you should follow the wife's way of calling.
If they have no children, oh well, compare their ages with your parents' ages, if they are more than 10 years younger than your parents, follow the husband's way.
I think you can call people 15 years older than you "anh", "chị" but it's always safer to call "cô","chú", especially when the older women are the ones to initiate that way of calling. I always follow the way of the elder people of the same gender as me when I view both options are ok. It also depends on whether the wife or the husband is dominant and I personally find it's better to please older women, who can be very strict, sentimental and detail-oriented, than older men who is (much) younger than your father.
Edited by Winged Rain, 27 November 2011 - 02:51 PM.