I think that in this case, scientists would have to confront Jeff to situations provoking big reactions from him and then do the same thing with the simulations to see which one is the real Jeff. It includes, for some people, to go beyond the limit of ethical actions to simulate Jeff considering you have to make him react to stressful situations like near death or pain without him being aware of it, just enduring it and reacting to it his own way.
Basically, in this case all you need to simulate someone is past events to refer to. It is true this method has limits, and works less in this type of cases, where past facts are blurred.
I also assume the simulations were done by machines and not by humans, simulating thousand of Jeffs at the same time with different " settings " and comparing it to the real world.
The fact that Jeff isn't reliable and has zero self-awareness can also be a data the machine could use to create the simulation.
I recognize you pointed a problem with my method, which if it, in theory, may works for consenting people who can enter the basic data themselves ( in this case the past facts memories ), is difficult to apply to people like Jeff without including a form of torture, this time in real-life.
The question of dead people is an interesting one too, this method being difficult to apply to deceased ones who, by their nature, don't have memories. Still working on it though.
I don't think we can quantify this pattern.
I see a solution to your problem but it may include a form of torture for the "other you".
Let's say you created a simulation of you, with some kind of feelings cursors you can modify ( humour : 77% - love : 56% for example ).
You, as a human, have a past and I will assume that you took that fact in count when creating the simulation of you, meaning your other " you " has memories etc... it isn't a simple you from the present, you can also simulate the 16yo " you " and see it behaving like you when you were 16, going in class, and not having the memory of something you did when you were 23.
I consider everyone has lived key events that changed forever the way he or she see the world in the present ( breakups, accidents, losses, wounds, crimes, wedding, deals... ), so all you need to do is take your " you " and put it in situations like the one you lived for real, to see if it behaves the way you did in real life. If your simulation reacts and behaves in these events the same way as you did, you could consider it's an excellent simulation of yourself and, if it gets hurt on the screen, you could feel bad for it.
To summarize, I think some things can't be quantify using numbers or some algorithms, and testing can be a solution.
Maybe exactly copying a Jeff is impossible, in fact I don't think we know all the personnality aspects someone can have so for now this task sounds difficult.
But in my opinion, if " Sim-Jeff " reacted to the same situation 2 000 000 times but with microscopically differences, it is pointless to keep that detail in mind. As a human being, you can't perceive those little behavior differences so it is useless to give them importance.
For example, Simulation n°1 320 678 SimJeff is scared and takes a 1 foot step back, Simulation n°1 320 679 SimJeff is scared and takes a 1.000306 foot step back. I think we can consider it's the same simulation outcome.
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As long as you enter a solid basic data in the computer like " I'm scared, I take a 1 foot step back and faint " - the computer tries to simulate the same situation ( trying to match the situation with your souvenirs as most as he can ) and when it comes that close that a human cannot see the difference between the SimYou and You, it can consider the simulation as realistic.
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It reminds me of and experience made. The drawing of a straight line is show to someone and he's told to draw the exact same one, then his drawing is shown to someone else with the same instructions, everyone tries his best to replicate the line from the person before him but each time they can't avoid to diverge a bit and when it comes to the 100th guy, the line is all twisted and crooked, even separated in two.
Maybe that is what we are, the 100th simulation of the original us who is way different than who we are now because every simulation diverge a bit in comparison from the previous one. But we'll hardly ever know it for sure.