In the Westminster Confession of Faith, we read the following in chapter 1 "Of the Holy Scripture" (emphasis added):
We may be moved and induced by the testimony of the Church to an high and reverent esteem of the Holy Scripture. And the heavenliness of the matter, the efficacy of the doctrine, the majesty of the style, the consent of all the parts, the scope of the whole (which is, to give all glory to God), the full discovery it makes of the only way of man's salvation, the many other incomparable excellencies, and the entire perfection thereof, are arguments whereby it does abundantly evidence itself to be the Word of God: yet notwithstanding, our full persuasion and assurance of the infallible truth and divine authority thereof, is from the inward work of the Holy Spirit bearing witness by and with the Word in our hearts.
Here in this well-known Reformed confession, the believer's ultimate knowledge is based on "the inward work of the Holy Spirit" that "[bears] witness" to the believer that the Scriptures (here, the Bible) is the authoritative Word of God. Sounds pretty familiar? Compare with two well-known texts:
But as it is written, Eye hath not seen, nor ear heard, neither have entered into the heart of man, the things which God hath prepared for them that love him. ut God hath revealed them unto us by his Spirit: for the Spirit searcheth all things, yea, and deep things of God. For what man knoweth the things of a man, save the spirit of man which is in him? even so the things of God knoweth no man, but the Spirit of God. Now we have received, not the spirit of the world, but the spirit which is of God: that we might know the things that are freely given to us of God. Which things also we speak, not in the words which man's wisdom teacheth, but which the Holy Ghost teacheth; comparing spiritual things with spiritual. But the natural man receiveth not the things of the Spirit of God; for they are foolishness unto him: neither can he know them, because they are spiritually discerned. (1 Cor 2:9-14)
Behold, I would exhort you that when ye shall read these things, if it be wisdom in God that ye should read them, that ye should remember how merciful the Lord hath been unto the children of men, from the creation of Adam even down unto the time that ye shall receive these things, and ponder it in your hearts. And when ye shall receive these things, I would exhort you that ye would ask God, the Eternal Father, in the name of Christ, if these things are not true; and if ye shall ask with a sincere heart, with real intent, having faith in Christ, he will manifest the truth of it unto you, by the power of the Holy Ghost. And by the power of the Holy Ghost ye may know the truth of all things. (Moroni 10:3-5)
While he is not Reformed by any stretch of the imagination, William Lane Craig (Molinist/Arminian/Evidentalist) did echo the words of Westminster in the following:
LDS apologist, Russell Ashdown, on his old LDS Website, critiqued James White's chapter against the LDS testimony in Letters to a Mormon Elder here. It is a good read.