Call the bank, confront her about it, if she's your wife, she should be open and shouldn't keep secrets. But, another option is ficus VERY closely on her spending and what she buys.
If people knows cause and effect then will never want to check on others. Whatever she does is due to karmic debts in past life meaning you have done something behind her back, so now she cannot change the way to make you think of her doing behind your back. And if you check into what she'd intentional or unintentional kept, next life she will do the same to you. Everyone can ask yourself if you want it to happen or if you want it to end it now in this life. Instead, gratitude and treating others from bottom of our heart can warm others' up and may want to be willing to share with you or cannot help to but share with you something never did before. Especially when people know what will happen as an effect because what he/she has done now.
From Master Lu who'd been helping people for free of metaphysics Q&A and live spiritual diagnosing in guanyincitta.com