Account Access Rules
If your wife has an account that is only in her name, then you cannot access that account without her permission. You may deposit funds into it, but legally the only person who can access, withdraw or transfer funds is the person authorized to sign on the account. If you attempt to get money from that account, her bank will notify her. To access that account, talk to your wife about making it a joint account. In that case, it will be your account too. You will have full rights to the same transactions as she does.
If your wife has passed away, you need to follow the bank's procedure for releasing the funds in the account and closing it out. Typically, the bank will require you to fill out documents and provide a death certificate. In addition, the bank will require some evidence that you are the person who should get the money. For example, your marriage certificate and her will may be required. Alternatively you can provide "letters of administration," which is a document given by a probate court saying you have been appointed as the person who will administer your late wife's assets.
Hidden bank accounts are common in marriages, and in the case of divorce, it is reasonable to want to know about your wife's financial picture. Although you may not have direct access to use funds in your wife's account, you can subpoena her bank records to see how much is in them and make determinations about support in a divorce case.
If your wife has a severe physical or mental health challenge, you may be able get access to her accounts through power of attorney or a conservatorship. This access is granted as a measure to protect her, not to unscrupulously use her money. While she is of sufficiently sound mind, she can sign a power of attorney document in front of a notary public and file it with your county's recorder office. To obtain a conservatorship, which grants significantly more authority, you must petition a court. The court will also provide ongoing oversight of your actions. Both statuses will allow you to make financial decisions and have access to your wife's bank account, but according to the Michigan State Bar, some banks are reluctant to grant access just on the basis of power of attorney. So although it requires more effort and expense, a conservatorship is a beneficial option.
The most direct way to access your spouse's account is to simply ask for it. Request she makes the account joint. Although there may be some private reasons for the account, you don't want to wind up in the legal snares that come with identity theft, the marital problems of sneaking an ATM card and trying to guess passwords and PIN, and the expense of trying to get a court order.