How do aircraft discern that an ACARS message is being sent to them?
A typical ACARS VHF transmission. 

Mode A 

Aircraft B-18722 

Ack NAK 

Block id 2 

Flight CI5118 &lt;&lt;&lt; looks like an identifier the receiver can filter on 

Label B9 

Msg No. L05A 

Message /KLAX.TI2/024KLAXA91A1 

https://en.wikipedia.org/wiki/ACARS