A variable that can be saved is any variable that is not declared as /global, /const, or /tmp.

/datum/a
    var/tmp/foo = "aaa"
    var/bar = "bbb"

/proc/main()
    var/datum/a/thing = new

    world.log << issaved(thing.foo) // 0
    world.log << issaved(thing.bar) // 1