DNA is a very effective way to identify an individual. Properly used it is almost infallible Only a few cells are needed for DNA identification,
The source of a DNA sample can come from any of saliva, blood, skin cells, sweat, hair or semen. What is important from this list is that DNA proves someone was at a location, but subject to such fact as may be shown from the source of the DNA (for example semen shows sexual activity), nothing else. DNA does not provide a timeline - DNA alone does not say when someone was at a location. Specifically, if skin cell DNA is found at a location of someone who already admitted to being in a location the DNA has proven nothing.