Hard to know how to answer this question with no context.
The earliest detectable signs of lung damage from smoking come in spirometry (lung function tests). Individual susceptibility to this kind of damage varies, but after twenty years of smoking reasonably heavily (20/day), you may well show some signs of airways obstruction. Eventually, this will get bad enough to be seen on a CXR in the form of lung field hyperexpansion, but that will take a while.
There are no signs on the CXR which can be said to be absolutely diagnostic of smoking, but putting together the clinical history and X-ray appearance usually tells the tale well enough.
QDM