Can I search trough my opponents graveyard anytime I want?
Yes, just ask first
I'm pretty sure I can, but my wife said I'm cheating so i need back up.
And they can't deny me to look through it?
The contents of the graveyard, battlefield, and exile zone are all open information. In addition the number of cards in a player's hand and library are also open information.
If you look at the graveyard you need to keep it in the same order. You cannot move the cards around.
Just to make sure things are official. You may look at your opponents graveyard, but it is polite to ask first. From the Comprehensive Rules:
400.2. Public zones are zones in which all players can see the cards' faces, except for those cards that some rule or effect specifically allow to be face down. Graveyard, battlefield, stack, exile, ante, and command are public zones.
RESOLVED