Let's open our Bibles to Romans 8:9-11.

If God’s Spirit dwells in you, you belong to Christ; and though your physical body will die, God will raise your body from the dead.

You are in the Spirit if the Spirit of God dwells in you, which is a mark of everyone who belongs to Christ
It is absolutely vital to have the Spirit of God dwelling in you, because if you do not, then you do not belong to Christ
There are distinguishing marks by which you can tell if the Spirit dwells in you:
Speaking in tongues is not a sign that the Spirit dwells in you
If the Spirit dwells in you, you have experienced the new birth
If the Spirit dwells in you, you are drawn to Jesus Christ, and you desire to know and honor Him
If the Spirit dwells in you, you have been flooded with God’s love so that you hope in Him
If the Spirit dwells in you, you regard Scripture as His word of truth and you are growing to understand it
If the Spirit dwells in you, His fruit is growing in your life and the deeds of the flesh are diminishing
If the Spirit dwells in you, you will have a growing hatred of sin and love of holiness
If the Spirit dwells in you, you will be growing in praise, joy, and thankfulness toward God
If the Spirit dwells in you, you will be growing in prayer
If the Spirit dwells in you, you will tell others about Christ
We who are in the Spirit are still subject to physical death, even though the Spirit has given us life
Death remains as the penalty on the human race until Christ’s work is consummated
We who are in the Spirit have the promise that He who raised Jesus from the dead will also resurrect our mortal bodies through His Spirit who dwells in us
Thomas Schreiner: The presence of the Spirit demonstrates that believers will not be saddled with their weak and corruptible bodies forever