Why are you all saying with out admin there is no group!.
What if i create a group and just leave ? I have seen groups with over million followers and there is no admin. May be admin Died, may be he left Facebook or just that group. Endless possibilities that a group could exist with out an admin.
Unless there is newer Facebook update that i am not aware of Which takes over groups that have no admin, sort of like reddit ?
Any ways. To OP. there is no one cut solution to know groups with out admin, You would have to manually figure it out.