$ udevadm monitor
monitor will print the received events for:
UDEV - the event which udev sends out after rule processing
KERNEL - the kernel uevent

KERNEL[4985.825951] change   /devices/pci0000:00/0000:00:11.0/ata4/host3/target3:0:0/3:0:0:0/block/sr0 (block)
UDEV  [4988.898331] change   /devices/pci0000:00/0000:00:11.0/ata4/host3/target3:0:0/3:0:0:0/block/sr0 (block)