It would be kind of hard to tell since they appear to be test fired at the factory.
IMHO I guess you can take it apart and see if the the bolt carrier is still fully coated with factory protective grease and not cleaned off and if the gas piston is only slightly dirty not black. If it has been fired 50 plus times the gas piston will at least be black, but from the factory it only has a little grime on it.
Best place to look is at the front of the bolt carrier where the gas piston hits. If it's been shot a lot, you will see some good wear there. Brass deflector is also a good place to look, however that part has been available and can be easily swapped.
There are a lot of private sellers claiming "never fired", it just shows their ignorance in the business they are trying to participate in. Never fired after original purchase would be the correct description.
Observation from a tour of the FN Herstal's factory in Belgium.
"Once assembly is finished, every weapon is test fired, for accuracy, reliability, and cyclic rate (in the case of machine guns)." Visit to FN Herstal