They state that those of the 144,000 who have already died, were resurrected sometime after 1914 and may be conveying messages to the Governing Body. The number will be complete at the time of the Great Tribulation and the rest, or remnant, will all be in heaven before Armageddon.
Anyone who died before Jesus are not part of the 144,000.
The Watchtower does not explain how anointed ones know they have been chosen to rule with Christ in heaven other than generic statements like 'they know beyond doubt' and you're not supposed to ask.