Were slaves ever put on trial?

[Flavius Inismeus]

Your post title at least can be answered - slaves were not legally 'people' under Roman law, so could not be tried. In the same way, slaves could not be legally raped or even murdered, as they were not 'people' - these were only crimes of property damage against the owner (in theory at least, I think - although as with many of these laws things seem to have changed over time).

I think it's an apt description of how things stood under the Republic. In most cases slaves were supposed to have been punished by the owner, perhaps after some kind of trial in 'family court' with amici as assessors. It is overwhelmingly probable that Crassus, Pompey, Octavian etc. executed rebellious slaves out of hand, i.e. without anything resembling normal judicial process.

However, even then there were exceptions when slaves ended up in court.

Valerius Maximus 8.4.2: 

On the other hand Alexander, a slave of P. Atinius, suspected of the murder of C. Flavius, a Roman knight, being tortured six times, denied to the end that he was culpable of the crime. It was as if he had confessed; he was condemned by the judges and led to the cross by the prison triumvir L. Calpurnius.

Dig. 48.2 reflects how things did change under the Empire - slaves were capable of being tried in courts of the Principate.

https://droitromain.univ-grenoble-alpes....ott.htm#II

(3) When an accusation is brought against a slave, the same rule should be observed as if he were free, according to a Decree of the Senate promulgated when Cotta and Messala were consuls.

(4) Slaves can be accused under all laws, with the exception of the Julian Law relating to private violence; because those who are condemned under it are punished by the confiscation of the third part of their property, which penalty cannot be imposed upon a slave. The same must be said with reference to other laws, by which either a pecuniary or a capital penalty is inflicted, which does not apply to slaves, as for instance, relegation. The Pompeian Law relating to parricide is placed in this category, because the First Section includes those who have killed their parents, their blood-relatives, or their patrons; which does not apply to slaves, so far as the provisions of the law are concerned. But as their nature is similar, they are punished in the same way. Again Cornelius Sylla was the author of the decision that a slave is not included in the Cornelian Law which has reference to injuries; but he is punished arbitrarily by a more severe penalty.

It is often said that under the Empire masters lost the power of life and death over their slaves to judges. However, the observations made by P.A. Brunt in “Marcus Aurelius and slavery” (M. Austin, J. Harries & C. Smith (eds), Modus Operandi: Essays in honour of Geoffrey Rickman. London, 139-50. 1998) must be noted.

In Gaius’ time the owner of a slave could charge anyone else who killed him with murder or sue him for damages (iii.213). Pius had also enacted that the owner who killed his own slave might be liable to the capital charge of murder (i.53). This was probably a restatement of existing law. Suetonius says that Claudius had forbidden owners to kill their slaves (Cl. 25); this shows at least that when he wrote the rule was not a recent innovation, still less due to Hadrian (HA Hadr. 18, a totally unreliable text).

Under Pius’ enactment, an owner was guilty only if he acted ‘sine causa;’ a contemporary jurist says that he could put to death a slave found acting criminally (Marcellus, D. xlv. 1.96, cf. Ulpian, xxx.53.3), e.g. attempting to murder the owner (Paul, xxix.5.6.3). We hear nothing of a iudex.