What you're missing is the Silph Scope which identifies all 'GHOST's in the Pokemon Tower as Gastly, Haunter and that Marowak on the sixth floor.
You obtain the Silph Scope by beating Giovanni at Silph Co. in Saffaron City...or is it at the Rocket Hideout in the Game Corner? Gosh sorry it's been so long.
Lonliness cries out into the darkness...but no one heard.
~Red Rum She Said