SWI-Prolog -- var

Availability:built-in

var_property(+Var, ?Property)

True when Property is a property of Var. These properties are available during goal- and term-expansion. Defined properties are below. Future versions are likely to provide more properties, such as whether the variable is a singleton or whether the variable is referenced in the remainder of the term. See also goal_expansion/2.

fresh(Bool): Bool has the value true if the variable is guaranteed to be unbound at entry of the goal, otherwise its value is false. This implies that the variable first appears in this goal or a previous appearance was in a negation (\+/1) or a different branch of a disjunction.
name(Name): True when variable appears with the given name in the source.