There are several ways to tell the difference at a glance from the outside. Front bumper is the usual give away as the newer ones have 2 slats and a dimple at the bottom, older ones 3 slats and no dimple. Headlights on the pre 06 have an orange stripe down the side. Other external differences are that the 06 models have LED lights on the rear and 07 onwards have a bonnet bulge.
Most aftermarket parts will fit all models, the main exception being anything associated with the front bumper (ie lips, spoilers, grilles and reflectors). You will see rear LED's, bulge bonnets and occasionally the newer style headlights on older cars as these are popular upgrades but owners of older Zeds rarely change the front bumper to an 06 version, they are far more likely to buy an aftermarket one.
Pete