You should try to gather as much information possible before a lawsuit is filed. During a marriage, spouses generally have equal rights to marital property. Each spouse has equal access to certain financial information, including joint income tax returns and joint bank account statements. If you live in a marital home, even if your name is not on the deed, it may be very easy for you to grant access to an appraiser to value the residence before filing. When a case is filed, things change. In divorces, when a case is filed, a special court order called an "Automatic Temporary Injunction" goes into effect. This injunction and the separations and actions that begin when a case is filed can limit your access to information. While your attorney can engage in a process called discovery to ask the other side to respond to questions, gamesmanship is common and their responses will likely be filtered by an attorney who sees their job as obstructing access to information.
In some circumstances it is simply not possible to gather all information that is needed before filing. You should still work hard to obtain everything you can on your own, because knowing what you have makes it easier to identify the things that you do not have and make special requests for discovery.
Every family law matter has its own unique informational needs. We have put together a list of information as a starting point to help people think through the needs of their situation and make a strong case.