there is no practical way to find whether the girl truely loves u or not. cheating is when u sign a contract and u dont deliver what u promised there. There are legal ways to fight cheating. Marriage is a very big decision and ashutosh i would want u and her to go together and take proffessional help to come to best decision. U dont need her consent to leave her mate but u might regret ur decision in future if it doesnt turn out to be the best so take guidance of a competent psychologist. u can go alone first and then as per the expert u can ask the girl to join.