Partridge matches the physical description both given to the CBI by a blind witness, and what we see in the episode where Red John saves Jane.

Partridge's voice matches Red John's voice.

Partridge pokes fun at Jane "whipping up the psychic ectoplasm" in the last episode Partridge appeared in, just as Red John did in the letter he left Jane after killing his family.

"The Tyger" is a poem by the famous William Blake, who also painted "A Brace of Partridge." The poem is either a reference to the fact that Red John is the Tyger and Jane is "The Lamb" (The Tyger's sister poem), or the fact that both Red John and Jane could in fact be the Tyger.

In the most recent episode, "Black Cherry" we see Jane's notebook of those he'd shaken hands with. The last page shown on screen has Brett Partridge listed on the left side with the following caption:

BRETT PARTRIDGE
CBI Forensics Dept
Totally Macabre
Worked on RJ cases


Lorelei Martins confirms that Jane has shaken hands with Red John, and Partridge being entered in this notebook confirms they had shaken hands.


With all of this evidence, it is pretty clear that Brett Partridge is indeed Red John.