Here's the thing about this question: The questioner assumes the Christian is making the judgement call.

Yes, the Christian is making a judgement call but only based on what and how the Scriptures have laid it out.

In other words, the Scriptures explainwhat is sin, therefore, the Christian compares what Scripture says and what people do which enables the Christian to say, "this is sin".

So who are we, as Christians, to say people are in sin?

It is God in His Word, the Bible, Who tells us we're sinful. It is us Christians who are repeating what God has already said.

If Scripture is anything to us, It is the foundation upon which we rest in our judgments. Scripture tells us, in no uncertain terms, we (humans) are all sinners. By nature. We are not sinners because we sin; we sin because we are sinners.  David tells us we are conceived in sin. Isaiah tells us even our best deeds are like polluted garments before God. And Paul explains, "for all have sinned and fallen short of the glory of God".

We are sinful people telling other sinful people that salvation is of the Lord. Martin Luther explains, "We are all mere beggars telling other beggars where to find bread."

Who are we, as Christians, to tell others they're sinful? We're sinners saved by grace wanting other sinners to be saved by grace. There is no other way to be saved. From our sin. From ourselves.

It is Jesus Who lived and died FOR us and in our place because of our sin, and we want others to believe in Him, too.