In general that isn’t easy. If there is reason to worry I normally create a loop and run it for a while, possibly using some memory leak detection tool such as valgrind or heaptrack.
For blob references it is typically safe to try and interact with them after the close and expect an existence_error. Some (older) code tests to represent pointers as Prolog integers and interacting with them after freeing can easily cause a crash. In newer code such reference are only used if it concerns stuff the user should never see.
Still, the fact that interacting with a closed stream produces an existing error doesn’t verify all associated memory and OS resources are properly reclaimed. The above loop is (I think) the only sensible way to test that.