Even much smaller aircrafts have some devices to check if the fuselage is still straight after a while or after suspected structural damages.
The ATR, for example, has got 3 triangular markers along the fuselage, and when you watch at them from the nose (or the tail) you must see them aligned.
I think they are used as the first hint only to do much more deep checks, and if there's really a structural damage it's gonna cost you the hell of a lot of money.
Some engineers will enlighten us on that.
