Here are a couple ideas.
Foremost, after a while you just get a "feel" for when a zombie is dead, or when he is still coming after you.
You know about how much damage it takes.
Note that different difficulties will require you to get different feels for this, so you'd need to play on the same one for a while.
Alternatively, you can switch your fire between a couple of zombies.
A bullet usually stops or at least slows down a zombie, even if he is going to continue rushing at you.
This gives you some time to deal with other zombies that may have gotten ahead in the rush due to this.
You can always come back to the first zombie later on if you discover he's really still alive.
Note that you can't rely on their "animations" per se because of the ragdoll physics.
If a zombie starts falling, he's dead, but quite often they take a full second or more before they actually begin to fall.
You can't rely on a certain pose or position to indicate that a zombie is dead.
gaming.stackexchange.com/users/1719/Invader Skoodge
Just for the record, this is supposed to add to the terrifying feel of the game (which I completely agree with)
However, if you raise the gamma a bit, then you will be able to see that the zombies start to fade when they die, even if they're still standing.
That's always worked for me.
gaming.stackexchange.com/users/320/Ritwik Bose
The best way I have found to be sure a lone zombie is dead it to try to walk through them.
A living zombie will stop you, but you can walk right through a dead one.
If they stop you, just push them back and shoot them again.
If you fighting a hoard, shoot at multiple layers of zombies.
When the front one dies, you will just start hitting the back ones.
gaming.stackexchange.com/users/1912/ILMTitan
I might be wrong, but they only "fade" in the censored versions of the game.
There is no way to be sure 100%, if a body shot has killed a CI or not.
The only way to be sure is landing a head-shot, especially with pistols and the assault rifle.
It's a whole different story in L4D 2 with the introduction of melee weapons, but I won't get into that.
gaming.stackexchange.com/users/4182/DrFish
Set your Left 4 Dead launch options to include -lv That way all killed zombies will fade away immediately.
gaming.stackexchange.com/users/14082/ordag
Count bullets.
Seriously, that's what I do.
It's not something I actively do anymore, now it's more of a feel but occasionally I still catch myself mentally counting.
As soon as the dead count is reached I just assume the zombie is dead, whether it's standing or not.
Because of this I prefer the AK47 (2 to body always kills), the magnum (one anywhere) and the snipers(one anywhere).
It's a little harder with the desert eagle, the trick is to not count each bullet but count each burst.
For me the m16 is just a bullet fest.
Obviously there are some caveats.
If I'm boomed and in a zombie rush
I often just empty my clip at head
level(unless
I've got a sniper).
Anytime
I see a headshot
the zombie is dead.
My technique doesn't work well in expert mode.
gaming.stackexchange.com/users/1317/deft_code
In Left-4-Dead, zombies don't always fall immediately when killed (especially with a pistol or machine gun), but instead there is a delay. Since I've currently been shooting zombies until they fall, I've been wasting bullets. How do you figure out when to stop shooting and when to move onto the next zombie?
How can I avoid wasting shots on zombies that are already dead, but haven't fallen in Left-4-Dead?
gaming.stackexchange.com
<left-4-dead>
gaming.stackexchange.com/questions/9365
gaming.stackexchange.com/users/1263/Casebash
The only way to be sure is with a head shot. Try raising the gamma so you can see when zombies start to fade.  There are a variety of ways that you can do this, the most practical way being just switching fire between zombies a lot. Counting bullets you have fired or making sure they are dead with a headshot also works. You can also tweak the in game settings by raising gamma and adding -lv to the launch options.
Users suggest switching fire constantly Users suggest tweaking the game's settings Users suggest ways in the game to know when one is killed