The suspicion of cheating can turn any mate into an amateur private investigator. And while it's not advisable to sneak around behind your mate's back to see if he's sneaking around, there are ways of doing it without getting caught. First, you'll have to prepare yourself for what you might find. Would it be easier on the two of you if you raise your suspicions in person? After all, it is the mature thing to do. Perhaps you already have and still have doubts. Either way, if you're ready to spy on your mate, then technology is definitely on your side. The most apparent ways to figure out if he's been engaging in salacious online activities include viewing the history on whichever Internet browser he likes to use. Check all browsers - names include Safari, Chrome, Internet Explorer, Opera and Firefox for those who are technologically challenged. If you see new software installed on your home computer, check it out and get a history of the Web sites he visits, if you can.
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.