We can tell the difference by looking at the composition. A primary atmosphere is similar to the Sun in composition. Bigger planets have larger gravities and can hold on to more atmosphere.
Smaller planets have lower gravities and can hold less atmosphere.
Temperature of the atmosphere . Molecules in a hotter atmosphere move faster than those in colder atmospheres, and are more able to esape the planet's gravity. Those in colder atmospheres move slower and are less able to escape. Composition of the atmosphere . Lighter molecules have less gravitational force acting on them, and are more able to escape.