There is really no way to determen what they have and when but just use the math would probably work. Take the amount of combos in the range of hand they would 3bet you with. Then look at the amount of combo that you would think are the top of his range. take the top range and divide into the total and that would give you the % your looking for.
It does matter how your opponent construct their 3 bet range. But 8% seems too value heavy and nutted. I would not 4 bet bluff and play cautiously against them.
I always recommend one rule to everyone, because when you sit at the table with opponents on the game, you take notes on each opponent and, accordingly, you will be able to analyze and understand when it is possible that he is bluffing.