If she were a sleeper agent, she would've been in the Federation for years. She'd have a perfectly acceptable background, because she'd be exactly what she appeared to be. There'd be no need to kill and replace anyone named T'Pel.
I'm sick and tired of people posting things like "T'Pel is a sleeper agent" without carefully working through the implications.
In a normal sleeper agent acting for X against Y scenario on earth, there are two possibilities.
First, an existing person from Y is convinced to help Y; lying low for a long time before betraying Y to X. Classic fictional examples: Manchurian candidate, Tinker Tailor Soldier Spy, or Philby in real life.
Second a person from X acquires a false identity and uses that identity to lead a quiet life, until he is activated and harms Y. Eye of the Needle is a good example from fiction.
Scenario 1 is obviously impossible because, among other things, T'Pel is Romulan, not Vulcan. She could hardly have pretended to be Vulcan prior to her recruitment, and would not have been able to afterwards since she'd been claiming she was Romulan.
So the second possibility must hold.
T'Pel must somehow have infiltrated the Federation and risen high enough to be given one of the must important diplomatic positions.
But when could she have infiltrated the Federation?
She could not credibly come in as an adult, because the Federation would want to know her lineage, her schooling, and would check with friends - the usual background check. Her position is prominent, and would be known to Vulcan, who would immediately want to know who she was?
Vulcans are famously obsessed with lineage and schooling, they would want all these answers. Even if false documents were given to the Federation, simple checks on Vulcan would reveal these to be false. For that matter, it would be hard to see how the Federation would not perform a deep background check on her, including interviewing all her old friends, typically from childhood at least.
The point is that an "infiltrator" type agent is typically used in a situation where the infiltrator knows he can fly under the radar. For example, maybe he is a low-level mechanic who will sabotage items when activated, or similar kinds of things. You can't really be an infiltrator when your story is so incredibly easy to check, and would be deeply checked both by Federation Security and Vulcan.
True, if she came as a child or maybe a toddler, again, there is the issue of her parents. In theory she could have been abandoned in an orphanage, with unknown parentage, at a very young age, and then later recruited - but how would she conceal her Romulan physical attributes? Is it really plausible that the Romulan would take some hapless Romulan babe, surgically alter her to appear Vulcan, abandon her to the Federation, then covertly monitor her activities for a seventy or eighty years (assuming Romulans age like Vulcans) or more, just in the hope she could somehow become Ambassador? If so, that's a more extraordinary story than the transporter trick!
Either way you look at it, it is simply not plausible that T'Pel was a sleeper. And I don't really see how she could have killed and replaced the real T'Pel either, although I suppose it is possible with enough hand-waving. But even there, it is odd that Picard never expressed any concern about the fate of the real T'Pel.
tl;dr The combination of (1) the highly secure nature of Federation Ambassador; (2) the very public profile of a Federation Ambassador both in Vulcan and the Federation; and (3) T'Pel's being of an anatomically distinct species without surgery from her cover identity's together rule out nearly any way for T'Pel to have been a sleeper agent.
