We've been told since TEOTW that the DO is Lord of the Grave. It is about time he started acting it. In [LOC: Prologue, The First Message, 59-61], we see "Aran'gar" and "Osan'gar," two minions of the Shadow brought back from the dead and put into new bodies. Osan'gar, at least, is one of the Forsaken; when he tells SH to stop choking Aran'gar, he thinks, "The thing had to obey one of the Chosen." [LOC: Prologue, The First Message, 60]. It is a good first approximation to assume that they are both reincarnated Forsaken, and not some random Dreadlords. So, what Forsaken are available for reincarnation (i.e. are dead)? Aginor and Balthamel, Ishamael, Asmodean, Rahvin and Be'lal, and possibly Lanfear. Well, Rahvin and Be'lal died of balefire, and their souls are beyond even the reach of the DO. Asmodean was a traitor; the DO lumps him in with Rahvin as having "Died the final death" [LOC: Prologue, The First Message, 15], and anyway, RJ has confirmed repeatedly that Asmo is thoroughly dead and won't be coming back.
Both the Gars were originally male. When Osan'gar tries to channel, he naturally reaches for saidin, not saidar. As for Aran'gar, she is extremely upset at being put into a woman's body, and Osan'gar thinks it is a "fine joke." Furthermore, in [TPOD: 16, Unexpected Absences, 333] Aran'gar thinks, "It was difficult now to really remember what life had been like as a man." This eliminates Lanfear. The remaining candidates are Aginor, Balthamel, and Ishamael.
Since the appearance of the Gars in LOC, we've met another recycled Forsaken, Moridin. From ample evidence in ACOS, TPOD, and WH, we know that Moridin was Ishamael (see section 1.2.3). Therefore, the Gars must be the Toxic Twosome, Aginor and Balthamel. Now our only question is which is which.
Osan'gar thinks to himself about having helped make the Trollocs and not liking the Halfmen who were an unexpected result of that experiment. We know that Aginor was very involved in creating the various Shadowspawn, and that he was unnerved by Fades, spending lots of time trying to discover how they "fade." In fact, from the Guide, we know that Aginor was the only one of the thirteen Forsaken to have worked on creating Shadowspawn [Guide: 5, The Dark One and the Male Forsaken, 50-60]. Therefore, we can conclude that Osan'gar is Aginor.
By a process of elimination, we can immediately conclude that Aran'gar is Balthamel. We have other evidence. From TEOTW and the Guide [Guide: 5, The Dark One and the Male Forsaken, 54], we know that Balthamel enjoyed "the pleasures of the flesh." That is to say, he was a real lecher. Thus, it would be a great joke to put him in a female body. Even more interesting, Aran'gar (Halima) now has a job "kneading young girls," as Mark Loy puts it-- something right up Balthamel's alley.
Finally, if all this wasn't convincing enough, we have a report from a post-ACOS book signing [Cincinnati, Ohio; 12 October, 1996]:
Someone asked RJ about the 'gars, and mentioned that he'd seen theories that Lanfear was one of the 'gars. I was expecting a RAFO, but RJ gave the guy a disgusted look, and said that "No, Osan'gar and Aran'gar are Aginor and Balthamel." The guy said, "You're confirming this, and not hinting about it?" RJ replied (I'm paraphrasing here), "I'm confirming. After all, it's pretty obvious in the books that it's those two. After all, that's what Aginor thought was so funny; Balthamel, the lecher, was stuck in a female body." [Mike Lawson]
The recycling of Aginor and Balthamel into new bodies did not come totally out of the blue. In [TEOTW: 50, Meetings at the Eye, 628], Aginor says, "Some of us are bound no longer. The seals weaken, Aes Sedai. Like Ishamael, we walk the world again, and soon the rest of us will come. I was too close to this world in my captivity, I and Balthamel, too close to the grinding of the Wheel, but soon the Great Lord of the Dark will be free, and give us new flesh..." (emphasis added)