1. You may know it from the testimony of the written Word.
"All things whatsoever ye pray and ask for, believe that ye have received them, and ye shall have them" (Mark xi.24). From this you know, that if you have, up to your light, fulfilled the conditions necessary to the filling of the Holy Ghost, on praying and asking for the Fullness, it is your privilege to believe that you have received what you have asked for; nay, it is your bounden duty, in compliance with Christ's express command, so to believe. If God gives, and you really receive, you may then give thanks, and that proves that you possess, for you cannot truly give thanks for what you do not possess! It will be noted that this answer is precisely similar to the answer that would be given to the question -- How am I to know that I am saved? By simple faith on the testimony of the Word. As multitudes have accepted salvation without any emotion, without any feeling whatever, so many a one has accepted by faith the "Fullness of the Holy Ghost," without any wave of emotion or feeling bearing witness to the fact of the filling. But this is not to say that there is never any feeling, that the emotions are never stirred; not so, for the feelings will come in due course, in God's own time.
2. Witness of the Spirit.
Again, one may know that the Fullness has come by the witness of the infilling Spirit. Just as in multitudes of cases the blessed Spirit bears witness with the Blood when it is applied at the moment of conversion, so many a one knows in his inner consciousness the moment when the Fullness of the Spirit was bestowed; they felt the Incoming and can date their Baptism, as others have felt the regenerating change and can date their conversion.
It should also be repeated here, that as many are ignorant of the date of their conversion, though well assured of the fact, so many may be ignorant of the date of their Baptism with the Holy Ghost, though well assured that they have entered on the blessed life. If we are assured of the fact, that we have received the Fullness of the Spirit, we need not worry as to dates.
3. Signs Following.
Yet again, one may know whether the Fullness has come to his heart and life by the signs following, by what "The Men" of the North of Scotland would call "the marks." Christ's words used in another connection may surely be applied in this, "By their fruits ye shall know them" (Matt. vii.20). "The fruit of the Spirit is love, joy, peace, longsuffering, gentleness, goodness, faith, meekness, temperance" (Gal. v.22, 23). The fullness of the fruit will surely be found where the Fullness of the Spirit is. Quantity and quality will both be there. As this has already been touched upon when considering the effects of the blessing, no more need be added here. But this, however, must be clearly borne in mind, that, while the Fullness of the Spirit is a gift, the fruit of the Spirit is a growth. Fruit grows, and the fruit will grow, if only we see to it that the conditions are present which are favorable to growth. That man does not manifest much wisdom who expects full growth without attending to the conditions of growth.