To be honest, I don't worry that much with old weathered driftwood, but if it's dry enough, sand it and:
a) smell it for a pine smell. If none, then even if it is pine, the pine resins have likely been weathered out
b) after sanding check for identifiable hardwood grain patterns like oak, cherry, etc. (or have someone who knows something about wood see if they can identify it)
--If these fail or don't reassure you, might be better to play it safe and not use it.