How to know when phone was last rebooted?

Occasionally I get the message about TouchID needing my password after reboot. Problem is, I haven't been rebooting. So I'm concerned my phone is rebooting itself in my pocket without me knowing it. I want to know for sure if my phone is rebooting by itself, or if the TouchID message is just erroneous. How can I find out if my phone is rebooting?

iPhone 5s, iOS 8 (latest)