Hash table element existence check

[Illuminati via Digitalmars-d-learn]

I am trying to create a hash table and would like an efficient 
way to be able to know if an element exists to test for 
collisions.

I could keep a bitarray, but wasting around 12% space. I could 
use pointers(null check) to elements but this creates 
fragmentation. It is not terrible, just curious if anyone has a 
better way?