When Raven enters Robin's mind and brief flashes of memories are shown, including a silhouette of Robin in a cave with his right hand raised (the crimefighting oath in the Batcave taken by Dick Grayson which is straight from Robin's very first story in "Detective Comics: #38) and Raven sees a brief moment of a man and a woman falling, with trapeze stands in the background (the death of the Flying Graysons - a major traumatic event which started the chain of events that led to Dick Grayson becoming Robin.). These are some of the most notable hints of this Robin actually being Dick Grayson.
Slade's illusory nature is hinted by his body never affecting his environment in the way others' do: In the downpour, Slade is the only one without a Rain Aura. As Robin chases him through the trees, the branches and leaves only move as Robin jumps on them. In a more blatant variation of the same idea, Slade skips around on the debris piles in his abandoned lair and they never budge. Robin lands on them, and they collapse. Whenever there's a flash of light, Slade alone disappears for an instant--indicating that he isn't just a hallucination, but one that only shows up in the dark.
The cracked mask that affects Robin is from the episode Apprentice: Part 2 (2003) - not the mask that was seen melting in lava from Aftershock: Part 2 (2004).
Many references to past Slade appearances on the series: His mask from Apprentice: Part 2 (2003), the 's' disk from Forces of Nature (2003), the Ferris Wheel from Betrayal (2004), and the broken gears from Slade's fallen headquarters.