It seems that Ellen White used the term "landmarks" (often proceeded by an adjective such as "old" or "ancient") in a general manner to mean, all truths "fixed by the word of God" (4SP 348.4). She explained, "Our only safety is in preserving the ancient landmarks. 'To the law and the testimony: if they speak not according to this word, it is because there is no light in them' [Isa. 8:20]" (5T 199.4). In reference to the last days of earth's history, White stated that "men and women will arise professing to have some new light or some new revelation whose tendency is to unsettle faith in the old landmarks. Their doctrines will not bear the test of God's word" (5T 295.3). When instructing parents on how to teach children "the old landmarks," she quoted Deuteronomy 6:20-25, indicating that the landmarks included "all these commandments . . . He hath commanded us" (5T 330.1). Many other statements, most of which are made in a general manner, support this understanding (e.g. LT 004, 1899; MS 003, 1899; GCDB March 2, 1899; 8T 160.2; LT 390, 1907).
It was apparently not her intention to ever provide an exhaustive list of the old landmarks. Nevertheless, she did mention a few specifics in MS 013, 1889:
"There is a bracing of the mind, an opposition of the soul, brought to the investigation of the Scriptures. This leaves such souls where Satan can impress them. In Minneapolis God gave precious gems of truth to His people in new settings. This light from heaven by some was rejected with all the stubbornness theJews manifested in rejecting Christ, and there was much talk about standing by the old landmarks. But there was evidence they knew not what the old landmarks were. There wasevidence and there was reasoning from the Word that commended itself to the conscience; but the minds of men were fixed, sealed against the entrance of light, because they had decided it was a dangerous error, removing the “old landmarks,” when it was not moving a peg of the old landmarks, but they had perverted ideas of what constituted the old landmarks.
"The passing of the time in 1844 was a period of great events, opening to our astonished eyes the cleansing of the sanctuary transpiring in heaven, and having decided relation to God’s people upon the earth, [also] the first and second angels’ messages and the third, unfurling the banner on which was inscribed, “The commandments of God and the faith of Jesus.” One of the landmarks under this message was the temple of God, seen by His truth-loving people in heaven, and the ark containing the law of God. The light of the Sabbath of the fourth commandment flashed its strong rays in the pathway of the transgressors of God’s law. The non-immortality of the wicked is an old landmark. I can call to mind nothing more that can come under the head of the old landmarks. All this cry about changing the old landmarks is all imaginary.
"Now at the present time God designs a new and fresh impetus shall be given to His work. Satan sees this, and he is determined it shall be hindered. He knows that if he can deceive the people who claim to believe present truth, [and make them believe that] the work of the Lord designs to do for His people is a removing of the old landmarks, something which they should, with most determined zeal, resist, then he exults over the deception he has led them to believe.
"The work for this time has certainly been a surprising work of various hindrances, owing to the false setting of matters before the minds of many of our people. That which is food to the churches is regarded as dangerous, and should not be given them. And this slight difference of ideas is allowed to unsettle the faith, to cause apostasy, to break up unity, to sow discord, all because they do not know what they are striving about themselves. Brethren, is it not best to be sensible? Heaven is looking upon us all, and what can they think of recent developments? While in this condition of things, building up barriers, we not only deprive ourselves of great light and precious advantages, but just now, when we so much need it, we place ourselves where light cannot be communicated from heaven that we ought to communicate to others."
Take note of the following statement from RH, December 29, 1896:
"Many know so little about their Bibles that they are unsettled in the faith. They remove the old landmarks, and fallacies and winds of doctrine blow them hither and thither. Science, falsely so-called, is wearing away the foundation of Christian principle; and those who once were in the faith drift away from the Bible landmarks, and divorce themselves from God, while still claiming to be his children. But are they?—No; no."
Take note of the following statement from 2SM 393.1:
"My soul is made very sad to see how quickly some who have had light and truth will accept the deceptions of Satan, and be charmed with a spurious holiness. When men turn away from the landmarks the Lord has established that we may understand our position as marked out in prophecy, they are going they know not whither."
Take note of the following statement from 3SM 406.2:
"After the truth has been proclaimed as a witness to all nations, at a time when every conceivable power of evil is set in operation, when minds are confused by the many voices crying, “Lo, here is Christ,” “Lo, He is there,” “This is truth,” “I have a message from God,” “He has sent me with great light,” and there is a removing of the landmarks, and an attempt to tear down the pillars of our faith—then a more decided effort is made to exalt the false sabbath, and to cast contempt upon God Himself by supplanting the day He has blessed and sanctified."
Since Ellen White considered all biblical truth to be the old landmarks, undoubtedly the list would include the major doctrines of Seventh-day Adventists. This would include the following: the Sabbath, the Sanctuary, the State of the Dead, the Second Coming, Righteousness By Faith (i.e., the Doctrine of Salvation), a literal Creation, the Godhead (Father, Son and Holy Spirit), and others.
If you desire to view all of Ellen White's statements regarding the "landmarks," please visit egwwritings.org and search for the term.