Of course Aegon is "the real deal." It's ridiculous the suspicions directed towards the validity of his Targaryen identity. I shall try to outline them now.
1. Firstly, there are the obvious physical and definitively Targaryen traits of violet eyes and white hair, in addition to his generally being exceptionally handsome and shrewdly intelligent, which nearly every Targaryen has demonstrated (perhaps, with the exception of Viserys).
2. There is the fact that the exiled Jon Connington, one of Prince Rhaegar's few close friends, is Aegon's protector and foster father, and has been since Aegon was a baby. If Aegon was a fake, why would a man so fiercely loyal to Rhaegar and the Targaryen dynasty endorse such an underhanded ruse and the tarnisihing of the family name with a pretender? If Aegon was a fake, Jon Connington would a) know and
would never agree to support such a lie.
3. Aegon is precisely the right age that he ought to be. How many young men of that specific age with white hair and violet eyes would one encounter on a day-to-day basis?
4. George R.R Martin has made clear throughout the books that there will be "three heads of the Dragon," three Targaryens remaining. People assumed Viserys, Daenerys and Jon Snow (perhaps Maester Aemon should Jon have been revealed not, in actuality, to be Rhaegar's and Lyanna's son, as is widely considered) would be the heads of said Dragon, the champions of the near extinct family. Fortunately, Viserys (that aggressively stupid gimp of a man) had a pot of molten gold dumped on him by his brother-in-law in a friendly family dispute gone wrong ("Khal Drogo do bad?"), so there was only Daenerys and, potentially, Jon Snow. Jon, Daenerys and Aegon make the three Heads of the Dragon as the series nears its finish- everything is inexorably coming into place in that respect.
5. People tend to say that GRRM would never have revealed such an important character as Aegon so late in the series if he wasn't destined to be ultimately discovered as a fraud. They also state that as he's conveniently not a POV character, so we can't know the inner workings of his mind and judge his authenticity from that. HELLO PEOPLE. The boy's character was unveiled late in the series as a sort of defibrilating, kick-to-the-nuts, outrageously unexpected and awesome surprise. SURPRISE! Gotcha, you know you jumped.
Anyway, as I was saying, Young Griff was meant as a surprise. We already had Daenerys fulfilling the role of the exiled last hope for the Targaryen dynasty. Both her character's importance, and the unexpected gravity of the significance of Aegon's character, would have been drastically diminished had they both been POV characters from the start. Admittedly, he is not a POV character, but neither was Robb Stark, Stannis Baratheon etc. GRRM uses other characters, such as Catelyn and Davos Seaworth as a medium through which we can observe the bigger, more interesting characters. GRRM uses Tyrion and Jon Connington as POV characters to observe Aegon, just as he used Catelyn and Davos to observe Stannis and Robb. It's a literary technique which GRRM has adopted to avoid the characters getting dull and create intrigue.
6. People say "oh Aegon can't be a real Targaryen, cos Daenerys was alerted by Quaithe to the existence of "the mummer's dragon", and so Aegon's a mummer and it's all a big mummery hoax." THEY'RE MUGS. THE OTHERS TAKE THEM. WORSE THAN FREYS AND BOLTONS. DEATH TO THE BOLTONS! DEATH TO THE FREYS! THE KING IN THE NORTH! THE KING IN THE NOR- sorry, got carried away there for a second. Anyway, if it had been "the mummer dragon," such aspersions would have been accurate. As it stands, he is "the mummer's
dragon." IT'S POSSESSIVE YOU PHILLISTINES. Varys swapped Aegon with a peasant boy when they were babies, knowing that Ser Gregor Clegane and Ser Amory Loch (two really friendly and sensitive guys. Ser Gregor actually enjoys knitting in his free time.) were approaching the Royal Nursery to murder the Targaryen Royal Heirs. Varys then gave the boy to Jon Connington to protect, and has been spending the rest of his life secretly working and plotting to undermine the realm's strength, to spread anarchy and sew discontent and mistrust throughout Westeros, all to facilitate the Rightful Prince Aegon's triumphant return. Varys, before becoming the Master of Whisperers, was a MUMMER. Aegon is "the mummer's dragon," as Varys saved him and has been steering him towards the throne ever since.
7. No one else could really be a replacement for him as one of the three Head of the Dragon, and the series is drawing towards its ultimate close.
Aegon is Aegon, guys.