Step 1: Create a True Name
The Storytellers Guide offers that each fallen's true name requires a number of successes to learn. Typical fallen require ten successes to learn their true name. Archdukes can require as much as fifty successes to learn. In game terms, we can equate this to the number of "syllables" the Fallen's true name contains.
Each syllable is then represented by a success earned in the game.
So in this example, Amon has the true name: .

Step 2: Create a True Name Grid
Create a small grid, with column equal to the number of successes needed. Each column then houses a syllable of the true name. Make sure there is space on the left side of the grid for the names of other characters who may be in the process of understanding which demon you are.
So in this example, Amon has the true name:

Step 3: Track using the True Name Grid
Each time a chracter, or an NPC gains the chance to learn the designated character's True Name, place a check or a dot in the area appropriate. Choosing a syllable can be random, or can be dictated by a dice roll (1d10 to figure out which of the ten syllables are discovered).
You can even opt to permit characters to find a specific syllable (say the second is what is really sought) by modifying the difficulty number. The point is, with this grid system, it is easy to keep track who knows what.

Bariel knows a syllable of Amon's True Name,
Janus knows two.
Note: Even for PCs!
The grid can even be used by PCs to keep track of which syllables of a rival demon, or even an Earthbound have been discovered.
By simply modifying the number of columns, you can keep track of true names much easier.
Don't overlook the fact that this system also permits new avenues for symbolism and artistic license. A syllable such as ø for example, can be utilized as a recurring symbol in the game session to hint at the True Name.

Amon has uncovered much of Janus' True Name,
Tierviel is bargaining the two syllables he learned for lessons in Lore of Patterns.