String message = "\\ABC1234\\"; // careful \ is an escape sequence so you double it to have this character


void setup() {
  Serial.begin(115200);
  for (size_t i = 0; i < message.length(); i++) {
    if (message.charAt(i) == '\\') {
      Serial.print("Found one \\ at index ");
      Serial.println(i);
    }
  }
}

void loop() {}