And this is what we see in scripture to see if we are in Christ:
"...love, joy, peace, patience, kindness, goodness, faithfulness, 23 gentleness, self-control; against such things there is no law. 24 Now those who belong to Christ Jesus have crucified the flesh with its passions and desires." (Galatians 5:22-24 NASB)
But, if we see us here in scripture, we may well not be in Christ at all:
"19...immorality, impurity, sensuality, 20 idolatry, sorcery, enmities, strife, jealousy, outbursts of anger, disputes, dissensions, factions, 21 envying, drunkenness, carousing, and things like these, of which I forewarn you, just as I have forewarned you, that those who practice such things will not inherit the kingdom of God." (Galatians 5:19-21 NASB)
Lack of spiritual fruit is the sign that you are in danger of hell fire:
"7 For ground that drinks the rain which often falls on it and brings forth vegetation useful to those for whose sake it is also tilled, receives a blessing from God; 8 but if it yields thorns and thistles, it is worthless and close to being cursed, and it ends up being burned." (Hebrews 6:7-8 NASB)
Circumcision, the putting off of the desires of the flesh, is the sign that you are in covenant with God. Just as putting off the flesh in circumcision was the sign of Abraham's righteousness and his covenant with God, so the circumcision of our fleshly nature is the sign of our righteousness and our covenant with God through our faith in the Son Promised to Abraham, Jesus Christ.
A big step of spiritual maturity for me was realizing that character is the measure of faith in Christ that counts, not ministry gifts and powers, or religious activities, or worship, but many people erroneously put their confidence in these kinds of things for their assurance of God's approval and pleasure in them.