The animorph books I read as a kid used < > brackets to indicate telepathic communication. It worked quite well, especially since the only character with it were the hero team and the big boss villain who would occasionally crash into their otherwise private communication.
I have scene books that use font/size changes, italics, and even bold letters, but those rely a bit more on quality printing (not as big a deal now as it is in those 80's paperbacks). I have also scene all caps used, but that was supposed to be an overpowering metal shout sort of thing, so it might not be too bad. Combinations can also work if a single doesn't click with you.