If you really want to get invasive - or if he's discovered the incognito windows some browsers offer - invest in a keystroke logging program that will record every letter he types. Of course you can scour his cell phone for suspect messages, and there's even some Bluetooth technology in which you can invest in order to track his calls and texts. Find apps that will track your joint spending and set alerts on them so that you know when money is spent. Many credit cards and banks offer these apps that give a detailed look at finances - and let you know when and where money was spent. Naturally, you can intercept those paper bills that come in the mail to peruse when you think he's using credit cards at places like hotels, flower shops or other inexplicable places. Just be sure you have a plan for when you catch him in the act - and a plan for if he catches you spying on him.