How to detect free/unfree memory segments?

[Suliman via Digitalmars-d-learn]

I would like to visualize how GC works and display free/not free 
memory segments.
How I can understand which of them are used and which not?
Could anybody explain what dangerous of memory fragmentation in 
languages without GC? Am I right understand that there is stay 
some small memory chunks that very hard to reuse?