Let's say it has to be the death of someone above the age of 30. They have to either volunteer or be a murderer. Since we're dealing with hypotheticals there is no way one could fake having a volunteer or murderer. Otherwise the ritual would not work.
The power received would be key knowledge that only the opposite side(s) have, given to those who perform the sacrifice.
There is no way of knowing what the information will be, only that it is very useful to them.
Would you be willing to use this weapon?
For the sake of argument let's say the murderer is in fact a murderer. There is no doubt at all. They took someone's life against their will. This is a not a stand trial kind of thing where there could be doubt in the form of mishandled evidence or crafty lawyers.