How does someone know if they have the Holy Spirit?

Answer:

"Now by this we know that we know Him, if we keep His commandments. He who says, "I know Him," and does not keep His commandments, is a liar, and the truth is not in him. But whoever keeps His word, truly the love of God is perfected in him. By this we know that we are in Him. He who says he abides in Him ought himself also to walk just as He walked" (1 John 2:3-6).

"And this is His commandment: that we should believe on the name of His Son Jesus Christ and love one another, as He gave us commandment. Now he who keeps His commandments abides in Him, and He in him. And by this we know that He abides in us, by the Spirit whom He has given us" (I John 3:23-24).

"No one has seen God at any time. If we love one another, God abides in us, and His love has been perfected in us. By this we know that we abide in Him, and He in us, because He has given us of His Spirit" (I John 4:12-13).

"We know that we are of God, and that the whole world lies in the power of the evil one. And we know that the Son of God has come, and has given us understanding so that we may know Him who is true; and we are in Him who is true, in His Son Jesus Christ. This is the true God and eternal life" (I John 5:19-20).