Hi Molecular.
As you know, the method of producing 'limited' wallet information from a bitcoin-core wallet has been proven to be 'safe' for the owner of the wallet. By this, I mean that if someone sends me the limited wallet information, there is literally no way for me to take possession of the coins.
So, in answer to your question... I have no way of being sure of my cut in this situation. In practise, I have been very pleasantly surprised by the number of people who are honest enough to send me my success fee when I have helped them recover their money. I would say that fewer than 15% have reneged on their part of the deal. Given that most people that contact me remain anonymous, that is surprisingly low. I guess that when you are looking at losing the entire amount of bitcoins, then to receive typically 80% of them back is pretty pleasing, and most do pay me the fee.
To add to this explanation, in fact most people these days just send me their wallet.dat file directly. I guess that I've been around long enough, and gained enough trust for people to feel comfortable doing that.
Plus other wallet formats (like armory, BitGo, airbitz, BIP38, Eth-pre-sale, etc) do not even have the ability to send 'trustless', limited wallet information, so those people just send me their wallet file.
I am well aware that my business model relies heavily upon trust and my reputation, so I ensure that I act honestly at all times with my clients (who now number over 900).
Cheers
Dave