Looking at the first paragraph of Poe's "The Pit and the Pendulum," there are several references making an allusion to the Spanish Inquisition, which was a nearly 400 year tribunal (1478-1834) of secular and religious nature, established by the Spanish crown with the Pope's blessing, that viciously violated Spanish law in its quest to find and subdue heresy and non-Catholic religious belief. An allusion is a literary device whereby a concept is given a broader and deeper meaning by associating it with a well-known historical, legendary, mythical or literary event, story or work. Today allusions are also often made to sports, television shows and films, as in "He's a terminator."
Since the Inquisition was a type of court where religious cases were heard and judged--sometimes on charges and in manners that were in opposition to standing Spanish law--any reference to judgement, courts, judges, sentencing, crimes, etc. may indicate an allusion to the Spanish Inquisition. How would one differentiate between an allusion to any court and legal proceeding and an allusion specifically to the Spanish Inquisition. One certain distinguishing element would be a reference with the word "inquisition" in it in one form or another.
The first paragraph starts out with a general allusion to imprisonment with "at length unbound me," but this general allusion stirs no association with the Spanish Inquisition. Nor does the next, which is "the dread sentence of death." It is the third allusion that clarifies the association with the Spanish Inquisition, though it does it indirectly, in true allusion style, and requires the reader to call upon his/er recollection of Spanish history: "the sound of the inquisitorial voices."
The allusion is continued with "black-robed judges"; "decrees of what to me was Fate"; and "the intensity of their expression of firmness—of immoveable resolution—of stern contempt of human torture." There is also "seven tall candles," which may represent the Jewish Menorah (candelabrum with seven candles) and may connect the persecution by the Inquisition of Jews in Spain to the rest of Spanish Inquisition allusion.