Examining Modifier Key Bit Flags 

The C64 is a very open platform. There is no memory protection and no virtual memory. All memory addresses used by the KERNAL have static addresses which makes them fun and easy to explore.
This video shows that there is a memory address, 653 in decimal, or $28D in hexadecimal, as you press modifier keys on the keyboard the bits in this memory address get flipped to represent the modifier keys held down. Simply printing this value out in an infinite loop lets you see the bits being flipped on and off in realtime as you press keys on the keyboard. Pretty cool.