var(X) succeeds if X is an uninstantiated variable.
Examples
?- var(abc).
no
?- var(1).
no
?- var(a(b,c)).
no
?- var([a,b,c]).
no
?- X=1, var(X).
no
?- var(X).
X = UNINSTANTIATED VARIABLE
yes
?- X=Y, var(X).
X = UNINSTANTIATED VARIABLE
Y = UNINSTANTIATED VARIABLE
yes
?- var(_).
yes