Do you use a balanced count?
We all have our hurdles. (My biggest one is remembering the running count when I return to it.)
If yours is deck estimation, and you cannot discern 6 decks from 8 decks, I would imagine that you would have a terrible time establishing the true count for balanced counts. I would first suggest that you use an unbalanced count.
As for a direct answer to your question, I recommend that you subscribe to either CBJN here or the conditions service on bjfonline. Both of these services accurately inform you of how many decks each casino uses (along with a lot of other good information).
Of course, you can always just call the casino on the phone, ask for the 21 pit, and ask how many decks are used.
In a case where a casino has both 6 and 8 deck games, I see no problem in slapping on your best ploppy face, looking the dealer straight in the eye, and saying, "How many decks do ya'll use in this here game?"