... where does the Bible command us to be assured of "someone else's" salvation
The first question that I typically ask someone who is lacking (to one degree or another) the assurance of their salvation is this, "is the way that you live your life now different in anyway from the way that you lived your life before you were saved?".
Also, does the life that you lead now (as a Christian) appear to be a life that is, at least generally/mostly, lead by the Spirit and focused on Him .. e.g. Galatians 5:22-23, or are you still walking in the flesh/according to the desires of the flesh (with all of its lusts and self focus) like non-Christians typically do .. e.g. Galatians 5:19-21
Appear, generally/mostly, and typically = subjectivity
Hath everlasting life = assurance
When you sin, are you grieved/sorrowful because of your sin, and do you immediately seek the Lord's forgiveness when you do, even for the smallest of your foibles/peccadilloes
Examine yourselves, whether ye be [saved]. How does that work? How do you determine whether you're saved, via self-examination?