I think the answer to this question can be found in the relationship between the cats. If the relationship is good and they are friendly with each other, they may play/fight especially if they are young. This will be ‘play’ because for a cat playing is fighting. If this play-fight gets out of hand one of the cats will complain and the other should stop. Young cats learn the limits to which their aggression should be subject when playing with other cats.
If, however, the relationship between the cats is not particularly good such that one is dominant towards the other or there are indications that there is a bit of stress between them then they won’t play with each other and therefore if they are fighting they are genuinely fighting and the animosity between them has spilled over to physical contact.
I think the above two points are the most important aspects of this subject. However, sometimes when friendly cats play/fight it does, as mentioned, get out of hand and it turns into a genuine fight. Sometimes siblings can fight with each other quite aggressively. It should stop but under these circumstances the motivator for ending up fighting is not aggression between the cats because they dislike each other but an innate behavioural trait within domestic cats to be individuals and establish their own territory.
When young cats become adult they have to establish their own territory and in doing so siblings can become rivals. Therefore what was once play can become a fight. In this instance I think you will see a difference in attitude; one which is more aggressive. In addition you should see a difference in general demeanour between the two cats in day-to-day life.
As I said, the relationship between the cats before the play or fight should provide a strong indicator as to whether they are playing or fighting. If you add to that the degree of aggression I believe you should be able to tell whether the cats are playing or fighting.
When adult cats living outside genuinely fight over territory it is extremely aggressive and quite scary to be honest because they fight in a way which can cause serious injury. Play-fighting is not as aggressive or as serious.
It might also be worthwhile to mention the style of play or fighting. When cat associates are play-fighting they are often in a comfortable position. They might be lying down poking and slapping each other and making lots of noise. But it lacks total aggression. When adult domestic cats are genuinely going at each other hammer and tongs it is quite obvious that they are fighting because the aggression is very evident and the atmosphere is very hostile.
The above are my thoughts on this but they’re not based upon scientific studies or the words of any expert. They are simply thoughts based upon general knowledge about the domestic cat.
If anybody else has some better thoughts I would be extremely pleased to hear them in a comment.