[The ruling on appointing a proxy (wakīl) to carry out qiṣāṣ]

The answer – and Allah is the One who grants success – is that the scholars of the madhhab have said: appointing a proxy to execute qiṣāṣ is not valid, except if the proxy carries it out in the presence of the one who appointed him; in that case it is valid. And it is in my mind that they explained the invalidity of (unrestricted) agency in this by saying: there remains the possibility that the one who appointed (the proxy) may pardon (the killer).
What appears to me is the validity of appointing a proxy, for the killer may be a fugitive, in hiding and fearful, while the guardian of the blood (walī al-dam) may be a woman, or someone chronically ill, or a weak old man, and none of them is able to track the killer, pursue him, and lie in wait for the opportunity to kill him. If we do not affirm the validity of appointing a proxy, the taking of qiṣāṣ will be lost and the killer will be safe.
This is supported by what people actually practice: some of the guardians of blood carry out qiṣāṣ from the killer while some other heirs are absent, in the courts and elsewhere, and none of the scholars has objected to that.
Yes, it is not permissible for the proxy to carry out qiṣāṣ unless he knows that the killer carried out intentional, aggressive killing – either by direct witnessing, or through the killer’s confession, or through shuhrah and tawātur, or by the ruling of a recognized judge.
Likewise is the ruling for the guardian of the blood: it is not permissible for him to carry out qiṣāṣ except in accordance with what we have mentioned regarding the proxy.
Yes, it is possible that knowledge of that may be obtained through signs and circumstantial indicators.

Source: Min Thimār al-ʿIlm wa al-Ḥikmah vol.2