They only presume to know because they assume that the 144,000 mentioned in Revelation is both a literal number and is the amount of anointed since 33CE.
The other problem is that they got rid of their teaching that there was a second class of Christians bound for heaven and this class was only identified around 1935 as staying on earth and use the feelings of some JWs who at the time, felt they had an earthly hope.
Revelation it's grand climax Chapter 20 states
Worldly commentators have interpreted this great crowd to be fleshly non-Jews converted to Christianity or to be heaven-bound Christian martyrs. Even the Bible Students in the past regarded them as a secondary heavenly class, as noted in 1886 in Volume I of Studies in the Scriptures, The Divine Plan of the Ages: “They lose the prize of the throne and the divine nature, but will finally reach birth as spirit beings of an order lower than the divine nature. Though these are truly consecrated, they are overcome by the worldly spirit to such an extent that they fail to render their lives in sacrifice.” And as late as 1930, the thought was expressed in Light, Book One: “Those who make up this great crowd fail to respond to the invitation to become the zealous witnesses for the Lord.” They were described as a self-righteous group that had a knowledge of the truth but did little about preaching it. They were to get to heaven as a secondary class that would not share in reigning with Christ.
Prior to the mid 1930s, from what I can gather, their teaching had been that their were two classes of Christians going to heaven. One being the 144,000 who qualified by their preaching work and a lower, second class. The rest of mankind were going to be resurrected to earth.
By ridding themselves of this second, Christian, heavenly class meant that all Christians prior to 1935 were going to heaven.
Wiki gives a bit of information as to some of the older teachings. I've tried to extract the relevant information from the article.
https://en.wikipedia.org/wiki/History_of_Jehovah%27s_Witnesses
1877 Book published in which Russell's concept of "restitution" — that all humankind since Adam would be resurrected to the earth and given the opportunity for eternal perfect human life.
Russell advertised for 1,000 preachers in 1881,[44] and encouraged all who were members of "the body of Christ" to go forth as "colporteurs" or evangelizers and preach to their neighbors in order to gather the remainder of the "little flock" of saints before they were called to heaven.
By 1904, Russell's doctrinal development was almost complete. His sixth and final part of "Studies in the Scriptures", The New Creation, established that Revelation 7 spoke of two heavenly classes of Christians — 144,000 who would serve as a royal priesthood with Christ and a Great Company who would be brought to perfection on a lesser plane, similar to that of angels, serving the 144,000.[
The earthly part of God's kingdom would be administered from Jerusalem in a re-established nation of Israel and under the control of the resurrected ancient Jewish prophets. All of mankind would over time be resurrected to earth in reverse order of death, Adam and Eve being the last, ..
n 1928 Russell's teaching that the natural Jews would be restored to Palestine and hold a prominent place in the earthly part of God's kingdom was dropped..
In 1935 a new interpretation of the "great company" of Revelation 7 placed them on earth as survivors of Armageddon rather than in heaven[156] and from that point converts to the movement were generally identified as those who, if worthy, would qualify for life on a paradise earth.