One should distinguish between a communication code, whose purpose is to enable a message to be clearly received and understood, despite noise, and encryption, whose purpose is to make the message impossible to understand unless you have the key.

Our radio, TV, and internet transmissions are coded in terms of characters, frames, and standard elements. Within these transmissions, the message itself may be clear or encrypted.

Therefore, in the hypothetical case of receiving a signal from ET, we might note regularity and stability of the carrier signal and regular division into "words" or "frames". This would identify it as an artificial signal.

However, if the message was encrypted, we might never decode the message!