What should one do who wronged people in the past, and does not know if they have forgiven him or not?

Repentance involves abandoning the sin, regretting it, and resolving not to return to it. If the sin relates to a material right of another human being, then it is obligatory to return the right or seek their forgiveness. As for non-material rights, the predominant view is that it is not obligatory to seek the forgiveness of the wronged person, and that it is sufficient to repent to Allah, especially if seeking forgiveness would lead to mischief. Therefore, repentance to Allah is sufficient, and it is commendable to strive in supplication and seeking forgiveness for those you have wronged.