He does this so he can judge whether or not his feigned madness is required any longer.
This is a clear sign that Hamlet is in fact only 'performing' as a deranged man, hence he is aware and in charge of his psychological status.
This shows that Polonius also finds this behaviour to be odd, proving Hamlet has changed or acted out of place - not normally.
If Hamlet had in fact been acting normally, then it would be fair to say that he is a natural 'madman' or faker, but he was not.
He goes on to find himself speaking of Claudius and how he plans to 'catch his conscience'.
This soliloquy is important as it shows Hamlet is beginning to form an obsession with retaliation and revenge for his fathers murder.
It would be a reasonable argument to say he develops a split personality.
Hamlet changes from a normal, well-liked man in Act 1, to a man using "madness" as an excuse for his flagrant cowardice in Act 111.
There is not even the slightest hint to show he was ever a faker.
One can only assume Hamlet has firstly, control of his mind set, and secondly, that he has used this change of personality as a vital role in his devious scheme for his fathers revenge.