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char myChar = '8'; // try with 'n' for a non numerical char

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

  if (isDigit(myChar)) {  // tests if myChar is a digit
    Serial.println("The character is a number");
  }
  else {
    Serial.println("The character is not a number");
  }
}

void loop() {
}