My father has passed away a few weeks ago. I miss him dearly. I am desperate to know that he is better placed, Allah had mercy on him, he is in Jannah and he does not have any regrets and sorrows in another world. I am desperately want him to be in best place.
Ordinarily, nobody is able to determine whether any specific person enters heaven. This is included within Knowledge of the Unseen (Ilm ul Ghayb). There's many Qur'an verses on this, e.g.:
Say, "None in the heavens and earth knows the unseen except Allah, and they do not perceive when they will be resurrected."
Tafsir al-Tustari writes of this verse: Thus, no one knows what has been predestined for him by God.
We're supposed to trust in Allah's forgiveness and justice, even though that's incredibly difficult when experiencing grief.
Hadith encourage us to pray, read the Qur'an, do good deeds, and give in charity on the deceased's behalf. See the fatwa Giving charity and reading Quran for the deceased. There's a few ahadith on this matter, e.g.:
When a man dies, his acts come to an end, but three, recurring charity, or knowledge (by which people) benefit, or a pious son, who prays for him (for the deceased).
Sahih Muslim 1631
A woman said: Messenger of Allah, my mother suddenly died; if it had not happened, she would have given sadaqah (charity) and donated (something). Will it suffice if I give sadaqah on her behalf? The Prophet said: Yes, give sadaqah on her behalf.
Sunan Abi Dawud 2881 [grade: sahih]