How reliable is an DNA test if you only have one parent and the child?
I'm guessing that you are asking about paternity testing. Paternity testing can be categorized by exclusion or inclusion. If the test done says that a man is NOT the father of a certain child, it should be 100% accurate.
The question then is how reliable is a test to see who is the father. Testing facilities provide an accuracy rating and it is highly suggested that if you want an accurate rating you go to a facility that consistently gives 99.99% accuracy. If the one parent you have is the father, you should be able to find a place that has a 99.99% accuracy rating.
If the one parent you have is the mother, and you are trying to determine paternity, you have a couple of options to keep a relatively high accuracy percentage. First, you could do a viability testing - which is a test on something biological from the father (blood, hair, other body tissues). If nothing biologically connected to the father is present, you can have a grandpaternity test done. This is where the parents of the father are tested.
Generally speaking, the accuracy of a DNA test increases with the number of loci (locations for DNA) tested.