ios - NSUserDafaults null value? -

i have code

[[[nsuserdefaults standarduserdefaults] valueforkey:@"identificator"]isequaltostring:@""] [[[nsuserdefaults standarduserdefaults] valueforkey:@"identificator"] iskindofclass:[nsnull class]])  

how can verify value key, or not. because code return no

this though.....

 nsuserdefaults *values = [nsuserdefaults standarduserdefaults];     if([[[values dictionaryrepresentation] allkeys] containsobject:@"yourkeytocheck"])     {         //return yes ...... or stuff here...     }