I've only started to play recently, but I have no trouble
serving aces with the slice serve (in future tournaments in the first year of my career on pro-10 difficulty - my serving skills are about 60% now, but I think you can serve a lot of aces after reaching 40% already). I've served 23 aces against a very slow player recently in a 6-0 6-1 match, and my average is about 10 per match, depending on my own form and my opponent. (Big, tall guys with high speed seem to be better at reaching good serves, although I could be imagining this...). I generally hit at least 1 ace in every service game, usually more.
Since there are no indicators,
it's purely based on timing, and apparently, different people go about this differently. My way is to
use the top of the net as an indicator.
First I
press and hold the accelerated shot button (b1+b2), and I
keep holding it all the way, not releasing it at all until the serve is finished. Then I wait for my player to throw the ball in the air, and
once the ball goes past the top of the net (still on its way up), I press and hold up+left at the same time. I wait until the ball reaches its highest point, starts falling back, and
release up+left just before it reaches the top of the net again, on its way down. The reason I'm using only left is because my player is right-handed, therefore his slice serve will spin further left, making it harder to reach for the opposing player. (On the other hand, slice serves are about 10 km/h slower than normal ones.) It doesn't matter which direction I'm serving from, I can hit aces from both sides.
It works the same way for a kick serve (down+direction), but I've never hit an ace that way. (Haven't tried too hard though, so it could be possible.) I can be useful however, to mix things up occasionally, I found that the AI is prone to make mistakes if you do something unexpected. (Once again, I don't have stats to back this up, but it certainly feels that way, or maybe I'm just seeing things...)
Hitting an ace without the slice works similarly, you obviously don't have to press and hold up, only the direction - the main difference is that because there is no spin added, you have to hold the direction button slightly longer for the serve to go wide enough and hit the line. (Either start pressing and holding before the ball reaches the top of the net on its way up, or release after it reaches the top of the net on its way down - practice it a bit, and you'll find the perfect timing.)
edit: I forgot to add something to clarify this further - I hold up+direction for slightly less than a second (I'm only guessing, but it should be approx. 0.6-0.8s), and I only release these buttons, b1+b2 button I keep holding!!! Also, using the top of the net as an indicator might not work for you if you have a different camera setting or a different serve animation!!! (I'm using default.)
by 
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