Well, DNA can be found anywhere where cells are present. This could be in the form of skin, tissue, blood, other body fluids. One thing to take note of is just because cells are present doesn't mean you neccessarily have DNA because DNA can degrade over time and depending on other circumstances such as temperature. Once a substance presumed to contain DNA, the DNA would be isolated via a procedure. The DNA may also be amplified with a PCR machine. This DNA would then be compared to the suspect's DNA (which would be obtained through a blood sample or saliva). It is a very complicated process and takes a lot longer to get a result than what you would see on CSI