Hi Tom,
Many of the most notable quotes I've found over the years came out of Lloyd-Jones commentary on Romans 8:5-17 when the doctor deals with Romans 8:15,16. I've found some others by Spurgeon from time to time, like this one from a sermon he preached on Acts 2:1-4: "Only the true Christian knows what it is to receive the Spirit, but there are only a few Christians who know what it is to be filled with it - to be filled to the brim." I believe that is true to the context of the sermon.
I guess I can copy and past a couple of other quotes I saved on my computer a while back (if its ok to do such things (I'm not too familiar with discussion boards). They refer to the witness of the Spirit.
Let me say now, before I turn from this point, that it is possible for a man to know whether God has called him or not, and he may know it too beyond a doubt. He may know it as surely as if he read it with his own eyes; nay, he may know it more surely than that, for if I read a thing with my eyes, even my eyes may deceive me, the testimony of sense may be false, but the testimony of the Spirit must be true. We have the witness of the Spirit within, bearing witness with our spirits that we are born of God. There is such a thing on earth as an infallible assurance of our election. Let a man once get that, and it will anoint his head with fresh oil, it will clothe him with the white garment of praise, and put the song of the angel into his mouth. Happy, happy man! who is fully assured of his interest in the covenant of grace, in the blood of atonement, and in the glories of heaven! Such men there are here this very day. Let them “rejoice in the Lord alway, and again I say rejoice.”
What would some of you give if you could arrive at this assurance? Mark, if you anxiously desire to know, you may know. If your heart pants to read its title clear it shall do so ere long. No man ever desired Christ in his heart with a living and longing desire, who did not find him sooner or later. If thou hast a desire, God has given it thee. If thou pantest, and criest, and groanest after Christ, even this is his gift; bless him for it. Thank him for little grace, and ask him for great grace. He has given thee hope, ask for faith; and when he gives thee faith, ask for assurance; and when thou gettest assurance, ask for full assurance; and when thou hast obtained full assurance, ask for enjoyment; and when thou hast enjoyment, ask for glory itself; and he shall surely give it thee in his own appointed season. – Spurgeon on Romans 8.30


Now sometimes the soul, because it hath somewhat remaining in it of the principle that it had in its old condition, is put to question whether it be a child of God or no, and thereupon as in a thing of the greatest importance puts in its claim with all the evidences that it hath to make good its title, the Spirit comes and bears witness in this case. An allusion it is to judicial proceedings in points of titles and evidences. The Judge being set, the person concerned lays his claim, produceth his evidences, and pleads them. His adversaries endeavoring all that in them lies to invalidate them and disannul his plea and to cast him in his claim. In the midst of the trial a Person of known and approved integrity comes into the court, and gives testimony fully and directly on the behalf of the claimer, which stops the mouths of all his adversaries and fills the man that pleaded with joy and satisfaction. So is it in this case. The soul, by the power of its own conscience is brought before the law of God. There a man puts in his plea, that he is a child of God, that he belongs to God’s family, and for this end produceth all his evidences, everything whereby faith gives him an interest in God. Satan in the meantime opposeth with all his might. Sin and law assist him. Many flaws are found in his evidences. The truth of them all is questioned and the soul hangs in suspense as to the issue. In the midst of the plea and contest the Comforter comes, and by a word of promise or otherwise overpowers the heart with a comfortable persuasion and bears down all objections that his plea is good, and that he is a child of God. When our spirits are pleading their right and title, He comes in and bears witness on our side, at the same time enabling us to put forth acts of filial obedience, kind and childlike, which is crying Abba, Father. Remember still the manner of the Spirit’s working before mentioned, that he doth it effectually, voluntarily, and freely – hence, sometimes the dispute hangs long, the case in pleading many years. The law seems sometimes to prevail, sin and Satan to rejoice, and the poor soul is filled with dread about its inheritance. Perhaps its own witness from its faith, sanctification, former experience keeps up the plea with some life and comfort, but the work is not done, the conquest is not fully obtained until the Spirit, who worketh freely and effectually, when and how He will, comes in with His testimony also, clothing His power with a word of promise, He makes all parties concerned to attend unto Him and puts an end to the controversy. Herein He gives us holy communion with Himself, the soul knows His voice when He speaks, there is something too great in it to be the effect of a created power. When the Lord Jesus Christ at one word stilled the raging of the sea and wind, all that were with Him knew that there was Divine power at hand. And when the Holy Ghost, by one word, stills the tumults and storms that are raised in the soul, giving it an immediate calm and security, it knows His divine power and rejoices in His presence. – John Owen (Communion with God the Father, Son and Holy Ghost) on Roman 8:16

Please pardon another from friend Spurgeon as such things are a wonderful to me.

I do believe that there is a supernatural way in which apart from means, the Spirit of God communicates with the spirit of man. My own little experience leads me to believe that apart from the Word of God, there are immediate dealings with the conscience and soul of man by the Holy Spirit, without any instrumentality, without even the agency of the truth. I believe that the Spirit of God sometimes comes into a mysterious and marvelous contact with the spirit of man, and that at times the Spirit speaketh in the heart of man by a voice not audible to the ear, but perfectly audible to the spirit which is the subject of it. He assures and consoles directly, by coming into immediate contact with the heart. It becomes our business then to take the Spirit’s witness through his Word, and through his works, but I would seek to have immediate, actual, undivided fellowship with the Holy Ghost, who by his divine Spirit, should work in my spirit and convince me that I am a child of God. – Spurgeon on Romans 8.16,17

Hope that helps. If you are really interested I'd get copy of MLJ's Romans commentary mentioned above where he goes into it in his characteristic depth.
Dan