Okay, I'm just drawing a complete blank here.  Does Buffy ever actually sense a vampire?  I know in Welcome to the Hellmouth or The Harvest, Giles talks about it to her at the Bronze.  But does she ever do it?  Or does she just always run into them?

And, is there any canon evidence that vampires can sense other vampires?  Particularly ones they're related to.

One time that stands out in my mind is in Season 5, when Buffy did the "spell to see spells," as she walks out of the house she gets partway down the sidewalk, pauses as though she senses something, then goes to the tree and yanks Spike from behind it, where he then tells her he was out. for. a. walk...bitch.

Another time was in Season 6, the beginning of As You Were, I think, when Buffy comes home from patrol and calls out "Oh, for crying out loud...Spike!"  And he steps from behind the tree (again, hmm), saying it was a fair cop, she caught him, but to be fair he wasn't actually hiding.

I'm also assuming the scene in Dead Things when they are on opposite sides of Spike's crypt door is implying that they can sense each other.  Wow, I guess the argument could at least me made that Buffy can sense Spike!

As far as related vampires in canon, I don't recall there being any evidence that they can sense their "family" members.

Plus, in "Pangs" Buffy keeps sensing Angel.  I think over a great deal of time, Buffy builds up a "spider sense" association with certain vampires, mostly Spike and Angel, while the rest of the time, she uses clues.  Like, for instance, the 70's dresser in "Welcome to the Hellmouth" or finding the college guy's copy of Of Human Bondage.  There is also evidence that Faith can't sense vampires, like when she thinks Spike is hunting a human girl, but it turns out, nope, she's a vamp.  Kendra also couldn't sense vamps, as when she attacked Buffy cause she saw her kissing Angel in vampface.  I guess it would also depend on situation.  Like, are there lots of other people around?  Noise?  Are the Slayer's senses already engaged, like she's recently been fighting?  Has she been recently institutionalized, perhaps put in a mental hospital by well-meaning parents, or turning herself in for murder after awakening from a 9 month coma?  Or perhaps she's not in tune with her senses/emotions, like Kendra or season 6 Buffy.  It seems like the ability to sense vampires, just as Giles said in "Welcome to the Hellmouth," takes training to reach its full potential. 

She appears to have an acute Spike sense, and a semi Angel sense.  It seems to me that they abandoned the original idea of letting them have the sense, because it could take away from certain story lines, I suppose.  In the film, she had a very painful vamp sense (like cramps to the extreme).

It seems like the ability to sense vampires, just as Giles said in "Welcome to the Hellmouth," takes training to reach its full potential.
I think you're right - hence the difference between Buffy and Kendra or Faith. Surely, with all her years of training, Buffy has learned to sense vampires (I'm sure it also helps that she's been intimate with two, thus making her hyper-aware of them). While Kendra had plenty of training, her actual experience as a Slayer was relatively short - and I'm assuming that the "spidey sense" is something that wouldn't kick in until she was actually called, so all her training as a potential wouldn't do much good in that area. And Faith, although she's been a Slayer for several years by season 7, doesn't have a whole lot of practice - she was in a coma for 9 months, and then jail for almost three years, so she hasn't had a lot of time to hone that sense, either.

Well, just started season four,  and Buffy is unable to detect vamps mere feet away from her (the vamp crawling out of his grave in the beginning, and the college guy she was helping with the map--she doesn't know he's a vamp until he turns around and is in vamp face).  I don't know about later seasons though--I honestly cannot recall.