Doesn't matter, the only way you can know who is truly saved is by their works.
No you can't. There are many unbelievers who live better lives than some Christians.
btw, where do you get the idea that believers are to figure out who is saved? For those who are viewed as believers, we need to be aware of what we observe. And as Paul noted in 1 Cor 5, if they stray, stay away from them.
9 I wrote to you in my letter not to associate with sexually immoral people—
10 not at all meaning the people of this world who are immoral, or the greedy and swindlers, or idolaters. In that case you would have to leave this world.
11 But now I am writing to you that you must not associate with anyone who claims to be a brother or sister but is sexually immoral or greedy, an idolater or slanderer, a drunkard or swindler. Do not even eat with such people.
12 What business is it of mine to judge those outside the church? Are you not to judge those inside?
13 God will judge those outside. “Expel the wicked person from among you.”
Paul clearly did not mean immoral unbelievers, per v.9-10.
btw, v.11 is wrongly translated. The words "who claims to be a brother" is in the passive mood, which doesn't mean they are claiming that, but they are viewed as a believer. iow, their behavior has been enough to view them as saved.