It's important for you to be confident in the group as well as the individual you use. If you've ever watched Rogue Traders, you'll remember that even with some of the professional companies, there are always a few scallywags who spend their lives taking advantage of the innocent, gullable, and vulnerable.
You 'may' be able to get a validated list of recognised PI's from your local constabulary. Failing that, your local newspaper may be able to assist you.
If your matter is civil, leave a note here and I'll expand on who you may wish to contact.
Good luck
Fr Bill