Not always. All forensics can do is is tell you how the person died and that's if there's enough evidence left.
In the case of Caylee, not enough of her was left so they (forensics) could not determine how she actually died. Even if she had been found a month after she died, it would have been difficult to determine how she died given that she died during the summer and she would have been mostly a skeleton in a month at the most.
Had she been killed with a gun and the bullet went thru her skull, there would have been good evidence. I believe Ted Bundy was in part convicted due to this sort of evidience. Bones in his case showed fractures.
Also, you would need an expert in ballistics to find out what gun was used in the murder and you would need to link that gun to the killer as well.
If she had bled while she was dying, forensics would have been able to collect evidence if the blood remained in her clothes and she was buried with them. I'm not so sure they could have enough of the blood sample to see if chloroform was in her blood though it's a possiblity.
But since lung samples and skin samples were needed to rule out a drowning, you need a relatively fresh body to get samples. By the time her body was found, it was mostly a skeleton.