If you would normally be able to access their documents in your normal day-to-day routine, this may not constitute spying.
If your spouse has left financial documents around the house or home office, you are allowed to make copies of these statements (as long as these are areas you would normally visit during your day).
However, the court may find that you were spying if you have gone out of your way to find and/or copy your spouse’s documents – even if you believe that they are lying about their finances. Frowned upon actions could include going through your spouse’s purse or briefcase, and in situations like this, you could be liable for “intrusion of seclusion.”