I've only dated two other guys before my current relationship, and my first was a childhood friend, as is my present. My last ex however was a bit older than me, ended up being a deadbeat, and one day robbed me of my valuables. I had almost $700 in cash to pay the rent.
If you have to go so far as to hire a detective or break his trust in you buy spying on him, there is obviously underlying problems that need to be addressed immediately. People tend to forget that it's communication that is key in a relationship.