another chelate question (3)

Besides the earlier mentioned 2 questions 
I have another:

3. What do Fe testkits detect?

_Do_ they detect _all_ Fe2 _and_ fe3?
Or is it impossible to detect chelated Fe?

For example: does the Dupla 

(don't say anything about costs here ;-) 
I just happened to get it from a friend..) 

Fe-test indicate all Fe?

And how about other testkits?