Dwarf hamsters can be notoriously hard to sex, a lot harder than syrians. A syrian male will have rather large "fun bags", just like my Munchkin. I would say a good 20% of his body is testicle!
Female syrians will be rounder at the bottom, and obviously have no testicles.
Another way to tell is by looking underneath:
The openings in female hamsters are closer together compared to the male.
Dwarf hamsters are harder because they are smaller, and usually faster, but a simple check of their underside reveals all:
The male of the species do have slightly visable balls, but it is usually easier just to check underneath.
Chinese hamsters are another breed who are clearly male or female due to their "fun bags":
Hopefully this post was educational for some :D