I have a bunch of theories.  Time will tell if I'm right.

1. The big clue that Heller was talking about is the fact that RJ is left-handed.

2.  Very few people know RJ's real name, including his minions.  Todd Johnson and Brett Partridge knew RJ by his nickname:  "Tiger, Tiger."  If you think about it, I always found it weird that the last phone call Craig O'Laughlin made was to Timothy Carter, NOT RJ.  You would think that a valuable FBI mole would be in direct contact with RJ; rather, O'Laughlin was corresponding with RJ via Carter and probably never knew his real name. 

3. Kirkland is trying to kill RJ for himself, probably because RJ killed someone close to him.

4. Bertram and McCallister are best friends who fought in Vietnam together.  The latter is RJ, and the former helps him out by feeding him valuable info.