The most telltale signs that it wasn't Clark

[litew8]

Originally posted by ClareKent
...when he said that "Kara was gone and he had to MOVE ON", I was like "WAIT, that's not Clark!!!" he'd never say anything about moving on

...or moving forward.

[SmallFarmGirl]

For me it was his devilish little smirk he gave when Lana said something like it's time for him to know everything that Lex is doing. Don't have DVR so can't remember the exact quote.

[kal-el returns]

Well, i didn't realize at first that it was Bizarro, until the very end. but after watching it a second time, the part where Clark and Lana are at the Isis Foundation Lana says to Clark, "Lex is the villain, not you." That was sort of a use of irony, which was referring to the true identity of the the current Clark(Bizarro).

[LoveHurts38]

When he flew to save Chloe and Jimmy was a surprise.

[alejandrita439]

For me the most telltale sign was Clark flying at the Daily Planet.
I was very surprised when Clark's face distorted into Bizarro so I never put it together during the episode but I was completely freaked when Clark seemed to fly up the stairwell to save Chloe. I kept saying to myself "did he just fly?"

i thought that too.. :P

[daxam77]

I just rewatched this episode last night. But I remember when I watched it the first time, I knew something was "wrong" with Clark from the moment he came onscreen, because they shot the scene with Lana showing only his back for a few seconds. That was odd. Then when the camera came around and you saw his eyes, there was that twinkle he gets when he's on red K or something.
Then the stairwell. I remember replaying that about thirty times, trying to figure out if he flew or jumped. You just can't tell.
But once you know, I must say the voice he used is so deep and wonderful. I'll bet Tom is a fine mimic.

[Ankhara]

I noticed too, that something was wrong with Clark. Something like a hunch. His behavior was different. He was somewhat cold. But the last thing that revealed that my hunch was right, was when Chloe says: "I missed you". Clark doesn't say anything, just smiles an awkward smile and looks away. I was like: OMG! that's not Clark Kent!

[Degobunny]

the blue jacket...

[xrayvision]

Clark has worn the blue jacket many times before Bizarro ever showed up.

[xrayvision]

For Lana the biggest giveaway should have been how he had sex with her when the real Clark never did unless he was powerless or Lana also had his powers.