I think the episode to check out is "And Now For a Word." This is the 36 hours on B5 episode done by ISN. In it, G'Kar is interviewed by ISN and they confront him with the claim that the Narn have not won a single engagement and that the war has devolved into a series of holding actions to slow the Centauri advance. G'Kar vehemently declares that this is Centauri propaganda, but I suspect that there was more than a grain of truth to it.
At no point in the series do we really get the feeling that the Narn are "winning" the war. Acts of Sacrefice is only a few episodes into the war and it is clear that things are already getting desparate, with Narn begging the other races to take a stand against the Centauri. The entire war only spans 12 episodes (roughly 6 to 7 months).
As far as what to believe, for myself, facts from the series always override the game books.
