If your board has no printed labels, check out its pinout diagram documentation (found on the board's Arduino Store page). PWM pins in the diagram have the same tilde sign in front of the pin numbers.

we start noticing some interesting components that are not commonly used on the Arduino. One of these things are the ICSP headers. They are those six pins that stick out, organized in a two-row ...

While we certainly do love the Arduino Nano ... using the bootloader, ICSP (in-circuit serial programming) and DebugWire are possible alternatives that connect the ICSP pins to the CH551 ...