Booting kernel from fastboot
When you use fastboot boot FILE.img, the image is downloaded and written into the RAM and than the normal procedure to boot a boot.img is followed. No changes to any partition takes place. 

If the image is invalid or cannot be booted into, the boot process automatically falls back to the image in the boot partition. Once the temporary kernel is booted into, it is treated no different from the one stored in the boot partition. You won't get to see "booted kernel" or alike info anywhere in the Android. Once you reboot the device that image gets flushed out of the memory and the one from the boot partition would be loaded and booted into.