For an authoritative overview of Salamandridae, see this page at the Tree of Life web project:
http://www.tolweb.org/Salamandridae/15445
Among the distinguishing characteristics, these experts note that:
* all newts have aquatic larvae
* all newts utilize ponds or streams to reproduce
* newts are highly poisonous in all stages of their life history
* newts have rough-textured skin that is not slimy
* with newts, costal grooves usually are not distinct
But as they point out, the subdivision of Salamandridae into "true salamanders" and "newts" is an
informal one.
This description makes me wonder though, which non-newt salamandrids are
not poisonous in all stages of their life history, and which newts
have distinct costal grooves?