Why does Hamlet feel it necessary to 'test' Claudius?

Hamlet learns at the end of Act 1 that his father was murdered by his uncle when his father's ghost asks Hamlet to avenge the murder. Hamlet does not actually act upon the command until Act 5, deciding in the meantime to first "put an antic disposition on," and then to test Claudius to assure he is guilty of the crime. Hamlet takes the time to observe everyone's behavior at the court and to establish that Claudius is indeed guilty because he is not impulsive—he is a thinker who considers the consequences and effects of his actions.
In Act 2, Hamlet says, "The spirit that I have seen/May be the devil: and the devil hath power/To assume a pleasing shape." Here we have the first reason Hamlet delays and instead attempts to assess Claudius's guilt. Hamlet understands that his father's ghost may not actually be his father's ghost but instead the devil or some other demon attempting to lure him into a sinful action. Rather than accept the ghost's word, Hamlet sets out to observe Claudius and test his guilt.
The second reason Hamlet resolved to test Claudius is the gravity of the sin he must commit. Hamlet knows that murder is an offense which will send him to hell for eternity. He does not want to be damned for the murder of an innocent man. The arrival of the players gives Hamlet the perfect opportunity to confirm Claudius's guilt. He resolves, "the play's the thing Wherein I'll catch the conscience of the king." Hamlet arranges for the players to perform a reenactment of his father's murder and his mother's hasty union. He tells Horatio to keep an eye on Claudius during the performance. Once Claudius's guilt overwhelms him as he watches the reenactment, he gets up and yells, "Give me some light. Away!" He runs off, confirming his guilt for Hamlet.
With the knowledge that Claudius killed his father and that the ghost spoke the truth, Hamlet is now able to clear his mind and avenge the death.