On the Pi, run

  vcgencmd get_throttled

You're looking for a 0x0 result (not now and never under-volted nor throttled). The result is a bitfield. A script to turn them into human-readable format is here but I just look at the bits: https://gist.github.com/aallan/0b03f5dcc65756dde6045c6e96c26...