This is how I would do it
char good[] = {"abcdefghijklmnopqrstuvwxyz"};
byte index;
void setup()
{
Serial.begin(9600); // Or whatever...
}
void loop()
{
index = 0;
while(Serial.available() && index < 26)
{
char aChar = Serial.read();
if(aChar != good[index])
{
Serial.flush(); // Dump the rest of the pending data
// Print out where the mismatch occurred
Serial.print("Bad character received when index = ");
Serial.println(index, DEC);
// Print out what the mismatch is
Serial.print("Received ");
Serial.print(aChar);
Serial.print(" instead of ");
Serial.println(good[index]);
// Get out of the while loop
break;
}
index++; // Advance to the next good character
}
}