The term "remastered" has become something of a marketing phrase similar to "director's cut". In most cases it means as close to an original master tape or file has been sourced and then possibly EQ'd, leveled and channel balanced. If there are previously unreleased tracks, then they may be "polished" to make them sound like the other finished tracks. Another important element of mastering, especially for vinyl releases, is track sequencing. How do you fit approximately equal amounts of playing time on each vinyl side? This can also be part of the remastering effort. Comparative listening is the only real way to tell what's been done. You can also contact the record company because they may provide you info on each release.
The meaning of “Remastered”
A lot of music I already have is being re-released and “remastered”. Some of those contain new tunes or printed material and I might buy (again) just to have that. Otherwise, what’s the value of a new master? I remember direct to disc vinyl was said to be limited to 10,000 copies because the “master” from which copies were pressed, wore out. Tape masters would have physically limited lifespans, too. But in the age of digital music, what is a remaster? I suppose a new release could have been “re-mixed” or “re-normalized”, so there may be real sonic differences which may or may not be an improvement. Does the use of the term mean there is some actual audible voodoo by an engineer rather than just procreation of an existing audio file?