Lu Xie wrote:

How did Nabiki figured that out?

The evidence:

1.  Akane was strangely defensive about the actions of Kaneda,
someone she supposedly didn't know.
2.  Ranma was first horrified and then amused by the sight of Kaneda
being kissed by a gorgeous girl.  
3.  Kaneda is supposed to be an old school friend of Ranma's.  
Ranma doesn't have old school friends, unless you count the 
ones who show up to try to kill or maim him.
4.  Ranma was suddenly (and successfully) avoiding cold water.
5.  Ranma got into a fight with someone named "Mizu" (water),
who gave him quite a pounding.
6.  Even when Nabiki investigated the situation by hiring Kouichi
to get Ranma wet, for some reason strange coincidences worked out
so that Kaneda always got wet instead.  (The mark of someone with
a water curse).
7.  Kaneda vanished, Akane reappeared, after Kaneda asked to use
the Tendo bathroom.
8.  She tried to throw the tea on Kaneda to finally confirm her
suspicion, but Ranma stepped in the way.  She then pretended to
be attracted to Kaneda just to see how appalled "he" might be.
9.  Akane has a very distinctively sweet smile.  So does Kaneda.  
Kaneda also has a build very much like Soun (who despite his emotional
fragility seems to be rippling with muscle everywhere as seen in
"Like Water For Akane").

Since Nabiki is supposed to be more than usually smart, I considered
this sufficient.  I'll send you the previous parts in a separate message
in case you missed these clues because you only just joined the action.

Oh, by the way, Kouichi didn't care about the money that Nabiki offered
him, since it wasn't much, and he's fairly wealthy.  He just figured
that Nabiki wouldn't respect him if he did her a favour without thought
of recompense.  

On Tue, 14 Oct 1997, David Johnston wrote:

Outside, she covered her mouth to muffle her giggles.
When she'd recovered, she said to herself, "Serves them
right.  Trying to treat _me_ like a mark.  Good thing I
figured it out though.  Akane does make a pretty
impressive guy.  For that matter, it's a good thing Kasumi
doesn't notice that sort of thing."