The 6th field of the GGA message will indicate if there is a valid fix or not (0=invalid). You may have to look into the TinyGPS library to see if/how that data is presented.
Similarly the 6th field of the GLL message is 'V' for invalid fix and 'A' for valid fix.
Similarly the 2nd field of the RMC message is 'V' for invalid and 'A' for valid.