Yeah I happen to live in a state that is very liberal in terms of what I can/cannot do to gather evidence. They don't even require PI's to be licensed, although the one I hired actually is.
Ultimately you need to be able to show through the evidence that you meet the standard of proof for adultery in your state.
The standard I listed would probably fly anywhere if you could establish that much, because most states do have common defenses (condonation, collusion, etc.) to adultery as well.
"You can't reason someone out of a position they didn't reason themselves into."
