Returns the next byte (character) of incoming serial data without removing it from the internal serial buffer. That is, successive calls to peek() will return the same character, as will the next call to read(). peek() inherits from the Stream utility class.
Serial.peek()
Arduino Mega only:
Serial1.peek()
Serial2.peek()
Serial3.peek()
None
the first byte of incoming serial data available (or -1 if no data is available) - int