Okay, so you're
in too deep. What exactly do you expect to prove? You might get very lucky and have the investigation come back with no evidence of whatever it is you suspect her of doing. Great, so your mind is eased for a little while, until whatever it is that causes you to doubt her begins to resurface. Then where are you? I bet it will be a lot like where you are now.
And if the results come back as bad news? What do you gain here if you are in too deep? More ammunition for arguments and fights? Best case is that you get angry and disgusted enough to quit her. Most likely you'll just whip up an emotional storm and then rationalize away the knowledge that you are acting foolishly, and continue the relationship simply because you're
in too deep.
If your heart controls your life, I sincerely have pity for you, and I am not being condescending. If you don't have the force of will to stop yourself from doing something you want to do that you
know is bad for you, blind luck is your only chance.
I make no comments about him or his services, but
http://www.willspy.com/ advertises to do just what you are looking for. Other CRT members have met him, I have not. If it works out and you end up living happily ever after, I hope you'll post the epilogue here. If it ends up badly, don't worry about posting, we've heard it a thousand times.