The first way I tell is by looking at the paint. Suspect body reair if the car has had any area repainted.
To tell about a repaint look for overspray in the joints, look at the surface for dirt (stuck in the paint), look at the surface for uniformity of texture (orange peel) and examine door jams/edges for tape lines.
Look for the decals that should be on the radiator core support and some hoods. Look to the fit of all panels. Look underneath for signs of fresh undercoating/seam seal. Look underneath for all new parts that aren't explained by maintenance.
Take it to an expert!
Bosch Master, ASE Master, L1
33 years MB technician