Going "all-in" might be largely subjective, but there are some reasonably objective standards that can be applied. Although a general manager almost always could have done a little bit more, there is a clear continuum with regard to how aggressively a GM is building a team. As a working definition, going all-in means assembling more talent for a given season or a set of years than a team would normally be able to do.
Three main ways of going all-in occur to me: 1) the way a team structures, re-structures contracts or extends players; 2) the way a team handles their draft picks; 3) the way a team uses free agency.