Sounds like you're asking a legal question, so you should address it to the legal experts.
But my therapeutic and personal experience with lying in couples is that partners tend to have very good intuition about romantic&sexual cheating UNLESS their family has a strong rule about "all men cheat if they can get away with it" or "women will flirt and cheat with socially alpha males as a way of protecting their interests and trading upward for their next marriage."
So if your intuition is about romantic disloyalty and not maxing out your credit cards, then I would ask you instead this: What would you do if you found out that your suspicions are more-or-less true? (Even if it's "only a little infidelity" like Internet Porn or cell-phone (or texting) flirtation with sexual content.) After all, it's a pretty good bet that he (or she) won't discuss it because s/he can't trust an "alternative explanation" to stand up under too much scrutiny. And kids and grownups lie when they're scared.
So consider what consequences you want to initiate when yo0u find out the truth, and then figure out how many of them to begin doing NOW.. And HOW and WHEN you're going to tell your sigothr what's coming soon if they don't come clean--so you can get started fixing the problem or separating your fortunes to restore a part of your own security and happiness without (him?).
PI is OK if there's a lot of money involved (in a divorce) and you need the evidence (for potential public humiliation) to strengthen your case.
Feel free to write back, but you're not offering enough money to warrant much depth of counseling. You'd have to offer 5 times this much to get an attorney to respond. So I'll offer additional services for double what you've deposited, and you can have this for your $5 but more depth--such as discussing how to stage you responsive actions to 1. maximize their impact in pushing your partner into coming clean and 2. still keep alive the possibilities that your marriage can be restored without irreparable damage, IF that is conceivable for you.