Who is the decider of the qualifications and their applicability in individual cases, will it be voters in a congregation, already established leaders in a denomination, or each individual deciding for him or her self who is a "true elder" that ought to be respected and obeyed?
Paul appointed Timothy and Titus to take care of this matter.
So, I do see hierarchy, here. But there is no naming of a certain organized group, simply the ones God had Paul appoint. Christians knew who their leaders were; they were family; the leaders took care of them as family > 1 Thessalonians 2:7&11. They knew who was who.
My opinion is they had the help of the prayer and recommendations of leaders already qualified; and if non-leaders were maturing in Christ they could tell the difference and help to commend new leaders.
You can tell who is ministering you to become more like Jesus and real in how the Bible says to love. I think this is what Paul meant when he said he knew the phonies were not real because they could not add anything to him > Galatians 2:6 > Paul says,
"they added nothing to me." This does not say they added nothing to what Paul said, but
"nothing to me."
What I understand is he could tell they were not ministering the effect of God's grace to make him more real in Jesus. But any child of God can minister God's grace, like this, as Paul has directed us to do >
"Let no corrupt word proceed out of your mouth, but what is good for necessary edification, that it may impart grace to the hearers." (Ephesians 4:29)
Peter could trust
"the multitude of the disciples" . . . the
"brethren" . . . to know who were qualified to be deacons > Acts chapter six. He told the
"brethren" to select men
"of good reputation, full of the Holy Spirit and wisdom". He knew they could tell the difference. So, my opinion is it was not by voting, but by unanimous consent, with prayer, of course. They did things as family.
But individuals, local churches, and whole denominations and organizations take things into their own hands. And we see the resulting anarchy . . . of minister's turning out to be predators and con artists, and spouses turning out to be abusers and cheaters, and pastors turning out to be burn-out cases. Because people are rushed through seminary and are evaluated by ones who can't tell the difference.
There are people who are not staying in the precise guiding of God's peace (Colossians 3:15, Proverbs 3:5-6) which keeps us in Jesus Christ's
"rest for your souls" (Matthew 11:28-30), and gives us God's sense to tell the difference.
If we obey how God takes care of us, we get taken care of.
And He organizes us along with the whole body of Jesus >
"in one body" > in this peace ruling in us > Colossians 3:15.
But people can take shortcuts in order to fill their pulpits, and in order to get married.
And, according to what I have seen in various churches and groups, there can be people who know what they are doing, even among others who are not doing things right - - - right in the same group or church or denomination. And people can tell who knows what he or she is doing.