First printing manga can be found on the inner pages of the book.
A numerical order display of 10 9 8 7 6 5 4 3 2 1 would indicate a first print while a 10 9 8 7 6 would indicate a sixth print.
First print manga were the originally released books before they were reprinted to later editions.
Older manga dating over a decade ago may have upwards of 20+ printings depending on the popularity of the book.
Later reprints may have corrections that were caught from earlier prints making earlier releases more rare as more reprinted books maintained consistency of any issues.
The older the manga, the more difficult it is to find a first print copy.