When someone else is offering to pay for your meal, ask your host for recommendations on what to order. This will give you an idea of how much your meal should cost. Photo by David Mertl.
In most social settings, when there is no commonly established etiquette, the traditional rule is "follow the host". To that end, if someone -- say a boss or an in-law -- offers to pay for you at a restaurant, the last thing you want to do is order a giant steak while your host sticks to drowning in unlimited breadsticks.