We know almost nothing about Spartacus before his revolt.

Plutarch's Crassus:

The first of these was Spartacus, a Thracian of Nomadic stock, possessed not only of great courage and strength, but also in sagacity and culture superior to his fortune, and more Hellenic than Thracian.

Appian (a little later than Plutarch) BCiv 2:

At the same time Spartacus, a Thracian by birth, who had once served as a soldier with the Romans, but had since been a prisoner and sold for a gladiator, and was in the gladiatorial training-school at Capua, persuaded about seventy of his comrades to strike for their own freedom rather than for the amusement of spectators.

That's the Loeb translation. I understand there's some controversy about the translation of the bit about being a Roman soldier. I don't have a problem with the Loeb translation but here's the Greek for people better at it than I:

ἐστρατευμένος ποτὲ Ῥωμαίοις

The Loeb doesn't give an ap crit; maybe a textual variant is where the translation becomes problematic.

Florus about the same times says:

the man who, from being a Thracian mercenary, had become a soldier, and from a soldier a deserter, then a highwayman, and finally, thanks to his strength, a gladiator.

These are all second century CE sources (maybe late first century), so quite removed from the revolt of 73 BCE. Florus only exists in epitome.

That's pretty much our info. It sucks that our most detailed source Florus exists in epitome. It both gives the impression that there was originally more info in the text and makes you doubt the info that exists, because we don't know anything about the epitomer, how good his text was or how well he understood it, and whether he imported information from other sources.

Sources agree he was Thracian, two of them that he was involved in the military and imprisoned, and one that he was a deserter and bandit.

I imagine one reason Spartacus is such an interesting historical character is that his background is so spotty; you can fill it in with almost anything you can imagine.

Why should we presume that Spartacus was a slave? None of our texts say so--in fact, Florus seems to presume that he was a free man:

Nec abnuit ille de stipendiario Thrace miles, de milite desertos, inde latro, deinde in honorem virium gladiator

Nor did he refuse them [the standards taken from the Romans that his troops presented to him], he who had been a tributary in Thrace, then a soldier [that is, he was called up as an auxiliary when his tribe was ordered to field a levy], and, having deserted the army, next a bandit, and finally on account of his strength[lit. "in respect to the honor of his strength," using honorem as an accusative of respect] a gladiator

Nowhere does Florus have him as a slave--indeed, his actions are those of a free man as far as Florus understands them. As a dishonored deserter (and a bandit, no less) it might have been perfectly acceptable for him to become a free gladiator--these are the sorts of people that Cicero mentions as gladiators in the Tusculan Disputations.

Anyway, that's what Florus says. Plutarch says he was in fact a slave, and says that a snake was coiled around his face while he was sleeping at auction and his wife, whom Plutarch claims was a Dionysiac prophetess (which sounds absurd to me, since Plutarch admits him a Thracian--he tries to backpedal by calling Spartacus a Hellenized Thracian, more Greek than barbarian, which I think is silly), declared this to be an omen of what a strong and cool dude he was. Plutarch doesn't tell us how he became a slave, however.

Appian also claims he was a slave, but unfortunately there's a problem with what he says:

Σπάρτακος Θρᾲξ ἀνήρ, ἐστρατευμένος ποτὲ Ῥωμαίοις, ἐκ δὲ αἰχμαλωσίας καὶ πράσεως ἐν τοῖς μονομάχοις ὤν

There was a man, Spartacus the Thracian, having once served as a soldier with the Romans, was, from captivity and sale, in among gladiators [i.e. he was one of them]

The problem with what Appian says is that it's unclear whose side Spartacus fought on. The gut reaction is to use the dative of advantage with Ῥωμαίοις, "he fought for the Romans," but that leaves the problem of how he ended up a slave. If that's what Appian means then Spartacus must have been captured by the enemy and sold by them (unless he really was a deserter like Florus says and was captured by the Romans and sold by them). If it's a dative of disadvantage, "he fought against the Romans," that would make a little bit more sense, but that's not the natural reaction you'd have when reading the passage. I've seen it translated both ways.

The short version is that, beyond the bare facts that everyone agrees on, namely that he was a Thracian, and was some sort of soldier at one point, we really don't know. Our texts generally aren't interested in how he became a gladiator (other than Florus, but that seems to be because he's just supporting his generally unfavorable view of Spartacus by saying that he was not only a free man--and thereby a gladiator by choice, which was not exactly reputable--but a deserter as well)

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