I never picked up on Reiner looking like their Titan forms, but the way Reiner kept misdirecting Connie and the constant reaction shots from Bert made them really suspicious.
I love how in sync the audience and the characters are during the reveal. It was becoming incredibly obvious that Rein and Bert are titans to both the characters and audience, and we're both getting ready for things to get tense, Reiner just says it and leaves everyone shocked.
I also love how them knowing and trying to capture Rein and Bert retroactively explains why that first conversation was so awkward and weird (Since there's no hole, I guess we should just go home RIGHT NOW and worry about the mystery titans later). It's stuff like that that show how well thought out the show is. It's the first anime in a long time that actually feels like a mystery, rather than just a show with mysteries in it.
On a side note, everyone keeps saying that Ymir is the one who killed Regime's friend, but they have different teeth. Flashback titan has flat, human teeth like most titans, but Ymir has razor sharp teeth like the Beast Titan. I doubt it's an animation error since they show the flashback twice. So either Ymir's titan form changed over time, or there's another Titan that looks nearly identical to Ymir.
A scene with both