Well,both my wife and I(who are quite old) have obviously been(and are) Godparents to our Grand Children.
No Vicar ever asked us if we were baptised.
I am not (despite being 86) my parents were anti religion and never had any of us baptised,this caused quite a stir when I went to school(but that's another story).
My wife (when I met her in 1947) was Jewish,but is now I suppose you would say Agnostic.
Have a look here:~
http://www.cofe.anglican.org/worship/liturgy/c ommonworship/texts/initiation/baptism.html
for the general wording of a Christening/Baptism..
If you can genuinely say these words,then the church would (I think) welcome you as a GodParent.
Of course,as other have said,it depends on the Vicar,but nowadays the church sems to be grateful for ANY new member (the baby that is), and if you help to further that process I am sure they will welcome you.