You can ask in three ways.
Is this my pencil?,(you are not sure whether this is your pencil)
This is my pencil,isn't it?(in this case,you think this is YOUR pencil)
This is not my pencil,is it?(in this case,you think this is NOT your pencil).
Note the reverse use of(is / isn't) and (is not / is)
You can apply this format to ALL type of questions.
Similarly you can use his or her.
Is this her/his pen?
This is her/his pen,isn't it?
This is not her/his pen,is it?
English is fun but too much variation.