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char myChar = 'A';  // try with 'a' for a non upper case char

void setup() {
  Serial.begin(9600);

  if (isUpperCase(myChar)) {  // tests if myChar is an upper case letter
    Serial.println("The character is upper case");
  } else {
    Serial.println("The character is not upper case");
  }
}

void loop() {
}