How can I tell whom a text message is addressed to? I have my iPhone 5S set up so I may receive text messages sent to my email account, my phone number AND my son's phone number. The problem is, when I get a message (say, one that was sent to my son's number) I can't tell if it was sent to ME or HIM. 

How can I see this info?
I'd guess you probably can't. 

The entire Apple/iCloud paradigm is to be able to get messages to you across multiple devices simultaneously.
To make it easy for you to deal with those messages wherever you may be. 

The last thought of the manufacturers/programmers was to differentiate message recipients inside one device containing multiple yous.