What Version of the Sketch did I leave on there?
First, you have to store the desired information in your sketch.

#define VERSION F("0.01 4-Apr-2019")

void setup() {
   Serial.begin(9600);
   Serial.print(F("Test Sketch V"));
   Serial.println(VERSION); 
}

void loop() { 
}


Of course any other way might work as well: 


Only print it, if at reset a test button is pressed
Check for a Serial command and respond the version info. 
Don't print it, but use avrdude and a programmer to read the FLASH memory and search it for the version string. (Similar to jot's comment above)