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I believe there is nothing within the charm person spell that explicitly make you forget what's happened to you. There is also nothing that gives a character a means to automatically be aware that they are under the influence of magic.
My ruling would be that it's very much a case of the character thinking back, going over past events and thinking "Hey I was behaving a bit odd then, maybe someone took magical control of me". It may be appropriate to ask the pc to make a wisdom check, with the dc depending on exactly how far they were forced to act out of character by the spell.
Chris