Well, if you or your friend can manifest a tongue of flame over thier head, it would be a good sign that you have the Holy Spirit and the grace that goes with Him.
But since neither you or I can do this, there is another way to be sure. If the spirit of God comes into contact with the spirit of a fallen man, one of three things can happen.
The fallen-nature (grace deficiant) spirit of the man somehow degrades God’s spirit making Him less godlike. (Not likely.)
A status-quo is maintained; neither spirit profits or lessens. (Also unlikely.)
The perfect and gracious spirit of God uplifts and ennobles the fallen spirit of the man raising him to a higher level than he was before. (This one gets my vote.)
Grace is a supernatural infusion by God. In essence, it is the act of bringing a man closer to God by the spirit of Christ, the Holy Spirit, or grace - whichever term you prefer - residing within the man the same way his soul was breathed into him by God in Genesis. If this infusion happens, the man will become more Christ like, rather than more fallen-like. He will become holier. This holiness will manifest itself in the attributes of his grace filled nature. Unlike his fallen self he will be more tolerant, merciful, considerate of others, less angry, vain, selfish, more charitable, loving, etc.
This is why the Catholic church states that works (charity) are essential to salvation. Not simply individual charitable efforts, but charity from a pure heart and from proper unselfish motives. Charity that is the fruit of a grace-filled nature. St. James says “You say you have faith without works, but I through my works will show you my faith.”
This is the answer you are looking for. When charity becomes “second nature” to you; offered for the one who receives from you with no thought of reward in return, then you know you have grace.