Getting status of a free variable.

Hello everyone,
Maybe it's very easy but I don't find anywhere a solution.

I need to know if a free variable is unified with any kind of value,
for example:

sillabo(Word,[H|T]) :- ....

sillabo(Word,[Last,Current | T]) :- ...


I want it to go to the second goal only if the actual list has already
a Last element that's unified with a value.
I'm sorry for my english and my prolog dictionary...
I'll try to be clearer if you don't understand my problem.

.