Probably ask your lawyer.
There's no logs of when a device was unlocked or what apps were opened. If they made or received calls, that can be shown in your call logs. The same with texts.
But, if you're wanting to prove if they opened Pokemon Go and transferred your powerful Pokemon back to the Professor, there's no way.
The time to have taken steps to protect your information was before your phone was confiscated. I've had people demonstrate their ultra-secure Tor network app on their cell phone that wipes out all trace of what they do when it's closed but had little personal interest in that level of security. I've been told (check with your lawyer on this) that a court can compel you to give the police your passwords. I've also been told that there are systems that allow you to invalidate your own passwords remotely, blocking your device. Again, I have little need for such high security.
Oh my, this is just horrible and I do hope things have worked out in your favor. The only way I know to protect your phone from someone accessing it is to use the built-in security features. Unfortunately, if apps and info have been accessed, there's no log that we keep that shows it.