How much that we can be confident of? Roughly as much as in the film John Wick (2014).

That is, the place where it's set is real -- John Wick in New York; the Iliad in Troy. The Iliad has at least one local point of interest that's real (the 7th century BCE cult of Ilian Athena); so does John Wick (New York harbor). We don't know for certain that the characters aren't real: in principle, they could be real. The events in the film aren't realistic but, in principle, they could have something to do with real events.

Actually, you know what? There's more material in John Wick that we can be confident in, than in the Iliad. The Russian mafia is a real thing, even if it isn't like it's depicted in the film; but in the case of the forces that Homer describes being involved in the Trojan War, we know that the groups he's describing are contemporary 8th-7th century BCE groups, not Late Bronze Age 12th-century BCE groups. Homer talks about the Trojans having allies like the Mysians, Kikones, Lelegians, Lykians, and so on; real Late Bronze Age Anatolia had population groups like Mirans, Arzawans, Assuwans, and Lykians. There's not much overlap.

The links posted by /u/gynnis-scholasticus are excellent for more info, especially the FAQ. The thread by /u/Iphikrates is admirably clear-headed. From my profile page, this one is also relevant.

Edit: some unfinished sentences

You might be interested in this answer by u/Iphikrates and this one by u/KiwiHellenist. The latter of these has even more on his flair profile, and there is also the FAQ