Usually viral infections generate an immune system which results in appearance of antibodies against these agents. Presence of these antibodies is used to tell if a person has been infected by a virus.
However, antibodies are detectable, generally, from three months since infection occurred. A good example is virus responsible for AIDS (HIV) antibodies generated against this virus become “visible” once passed what is called the window period, usually 3 to 6 months.
There are methods that detect infection at early stages, all are based on PCR. This technique allows us to amplify small amounts of genetic material portions of the virus to make it detectable. Its use is established as routine in the detection of virus in blood donations, in neonates and in those who are under medical treatment and need to know how many copies of virus that have (viral load).
PCR has great advantages: high reliability, speed and early detection of infection. However, it is a technique that has ALL as an error range. In his case, error range is more inclined towards occurrence of false positives false negatives. Therefore it is considered very reliable in determining that a person is not infected. In the case of a positive PCR result, we recommend several subsequent studies for verification. These later studies also includes new PCR and what we call Western blot. This technique checks the existence of virus proteins, but it has disadvantage that it needs to be considerable to detect viral load.
In summary: For recent infections most effective method remains the PCR, whereas ELISAs and Western Blots are reliable once you have passed window period of three months.