Well, for example you can connect a serial-to-usb adapter and continuously poll the log while the device is running. If this is a recurring issue, we may have a chance to see what was happening before. Other option is ADB via Ethernet, which allows pulling the logs and shows you either the whole device is frozen or just the screen output. I have my RPi3 running for more than 4 days now and its still working fine. If you know how I can reproduce the freezing, please let me know.