In addition to Bertram's tea-drinking, Wm. Blake poetery-quotes, age and general physique consistent with what we know about RJ, there are more subtle clues that suggest Gale Bertram is RJ:
In the episode where Jane meets Gale Bertram and shakes his hand, (Season 3, ep 1) Jane (wrongly) suspects Bertram could be the killer in that episode. Mid-episode Jane states (regarding Bertram) "I got a weird vibe the first moment I met him." In Season 5, Lorelei states that she is surprised that Jane did not know RJ when Jane met him and shook his hand. Jane did sense a "weird vibe" upon meeting Bertram to suspect he could be a killer.
In Strawberries and Cream (Pt. 1), after Gupta is killed, Bertram tells Lisbon there is no need to pursue the purpose of the money transfer "list" - the list that would ultimately inform the team that RJ was behind Gupta's crime and was seeking Hightower. When Bertram is informed about the details of Gupta's scheme he calls it "elegant." RJ would consider his criminal plots to be elegant.
in "Strawberries and Cream (Pt. 2) the script stresses that all but one person on the list as possible RJ accomplices are innocent. Bertram was on the list - but we know O'Laughlin was RJ's accomplice. It seems like purposeful misdirection by the scriptwriter to state that the others are innocent - more than one on the list could be RJ and/or his accomplices.
RJ would want to work closely with Jane to observe him - RJ obviously considers himself to be better than Jane - what better role for RJ than as Jane's superior?
