Investigation described simply, is drawing conclusions from observation. You should now have some observations made by looking at your monthly cash flow sheet from earlier. Did you spend Waaaay too much on dining out? or did movie rentals eat a lot of your money. Video Games for the Kids? Perhaps you have noticed some trends, does your husband always get what he wants? or do you just put it on the credit card because it seems more convenient?
Pay attention to how you spend money everyday. The situations, and what motivates the purchase. The more you observe, the better conclusions you will make. What are you triggers? When the kids whine and cry in the grocery store, do give them what they want just to placate them? Does your Husband make online purchases and forget to tell you? Observe everything.