Prolog type checking

Prolog defines a group of built-in predicates for type testing purposes: var/1, atom/1, integer/1, float/1, atomic/1, compound/1, nonvar/1, number/1, all of them with quite a self-explanatory meaning if you know the data types of the language. For specific characters, you may exploit unification with that character, after checking that the element is not a free variable (otherwise unification is always successful).

You could try this code:

isList([_|_]).
isList([]).

Hope it helps.

To check if a variable is bound to a list, you can use is_list/1.