For the quick skills you'll really only be able to intterupt them by anticipating their use and using the interrupt just beforehand.  The average human reaction time is something around 0.2-0.25 seconds, and the game apparently standardizes everyone to 350ms lag time, so that's more than Sedna's 0.5-second cast time for Heal.  And forget about interrupting 0.1-second skills just by watching for them, that's utterly impossible for a human to do so those you'd have to anticipate.

To put the interrupt window into perspective:

http://getyourwebsitehere.com/jswb/rttest01.html

*ahem* 350ms is .35 seconds..so provided your cast is faster than .15 seconds on the interupt you'll get her.. and oh look torchie is .1 ...Foul grasp is instant...

And yes anticipation helps...

Do interrupts wait until after the current autoattack cycle to cast?  I noticed as Oak that Penitence activated after I swung my axe.

for skills with a cast time of .5 or less it is normally just a luck and experience thing needed to interupt. I have a friend that can interupt sedna's heal plenty during matches. He just times when it should be recharging back up and when she'd likely use it. Seems to work for him.

Hence the "skill" in anticipation and reaction time to get it just right...

If you want to reliably interrupt, stick with the long cast times. Like rooks hammer, regs mines etc. Sure, of course, i once interrupted a heal, even a rain of ice, but that was either dumb luck or pinpoint anticipation (sedna was at 25% health, of COURSE she whas gonna heal). But don't be too hard on yourself if you miss an UB spit.....

Dunno about interrupting sedna, but I played a game night before last where I was fairly consistantly interrupted and chain stunned as rook.  My abilities are fairly slow cast so it's relatively easy for players to do that, especially with scrolls and potions.  So in a very general way, interrupts are possible and highly effective, especially UB's