The Bible makes it clear that we are required to make our election and calling sure (2 Pet. 1:10). We can have the assurance of being elect.
But if I am not yet a believer, that doesn’t mean I will not be tomorrow, or the next day, or even on my deathbed. So I cannot possibly know in this world that I’m not elect.
From a practical sense, I assume that every person I ever meet is elect even though I know it’s extremely unlikely that they would all be elect. My working assumption in evangelism and outreach is to be hopeful the person to whom we’re speaking is numbered among the elect.