A scientist could look for spores to identify a plant as a club moss rather than a liverwort. While both club mosses and liverworts reproduce using spores, club mosses have specialized structures called strobili (cone-like structures) where the spores are produced, which are not present in liverworts. Additionally, club mosses belong to the group Lycopodiophyta and exhibit characteristics typical of vascular plants, whereas liverworts are non-vascular plants (Bryophyta).
The other structures mentioned do not provide definitive differentiation, as both groups can have rhizoids, and neither group has true flowers or phloem in the way flowering plants do. Hence, spores and their production structures are the key identifying feature.