Exactly the problem!
You are not going to say - that he is dead,right?
But you don't know if he is alive.
So ,what now?

Exactly!

Well,the _ donation _ is not a problem .The problem is :
When do you consider a person dead ,so you can use his organs? In other word ,the moment of death.
This is _ not _ a simple question to ask.
The current knowledge in medicine says the moment of death is the "brain death".
However this is not so simple.
If there _ is _ a possibility of a person to be back alive after a "brain death" then taking his organs is
pratically killing him ,right?
This "moment of death" is the real Judaism problem .
While doctors say it is OK to take your organs ,how do you make sure of two things:
1. The person is _ really _ dead (when we think of a person as dead?)
2. Why would you trust _ any _ doctor on such a thing?
So ,there are two main opinions in Judaism:
1. The moment of death is the brain death ,_ but _ to make sure that person is really dead
      there is a need of more then just one doctor to decide .Let say we need two doctors to say
      that and one _ none _ doctor to see that the two other guys are not going to just kill somebody
      for money.
      There is a law in preparing actually in Isarel that makes all this to happen.

2. The moment of death is the moment of heart actually stopping.
        This is _ against _ of current science ,however I would not trust a science for this, science changes.

Note however that if an organ was _ already _ taken than the person is already dead anyway and thus
the "receiving an organ is OK under religious law".
Just trying to clear thing out.

Well,the _ donation _ is not a problem .The problem is :
When do you consider a person dead ,so you can use his organs?
This is _ not _ a simple question to ask.
The current knowledge in medicine says the moment of death is the "brain death".
However this is not so simple.
If there _ is _ a possibility of a person to be back alive after a "brain death" then taking his organs is
pratically killing him ,right?
This "moment of death" is the real Judaism problem .
While doctors say it is OK to take your organs ,how do you make sure of two things:
1. The person is _ really _ dead (when we think of a person as dead?)
2. Why would you trust _ any _ doctor on such a thing?
So ,there are two main opinions in Judaism:
1. The moment of death is the brain death ,_ but _ to make sure that person is really dead
      there is a need of more then just one doctor to decide .Let say we need two doctors to say
      that and one _ none _ doctor to see that the two other guys are not going to just kill somebody
      for money.
      There is a law in preparing actually in Isarel that makes all this to happen.

2. The moment of death is the moment of heart actually stopping.
        This is _ against _ of current science ,however I would not trust a science for this, science changes.

Note however that if an organ was _ already _ taken than the person is already dead anyway and thus
the "receiving an organ is OK under religious law".
Just trying to clear thing out.