Sex of hamsters

Question
I bought a campbells dwarf 3 weeks ago.  He or she is probably about 2 months old now.  At first I was told this hammy was a male.  I went to get him a friend and was told that my hamster  was a female.  I am not conviced and I really need to know as I bought another hamster and they are getting along...but I do NOT want babies.  My hamsters' name is Buddy and he appears to me to have the "big butt" that I assume is his testicles.  When I looked for his vents I saw that they were far apart..but yesterday and today the top vent was bulging some.  Also he is sniffing the other hamster (who is probably about 4-6 weeks old)  near the vents and then runs off with his (?) tail sticking straight up.  How can I tell if these hamsters are male or female?  

Answer
Hi Helen,

The best way to tell what sex your hamsters are is from a picture.  Here's a link to a good, clear picture:

http://www.hamsterhideout.com/forum/index.php?showtopic=10517

If you have any other questions, please feel free to ask.

Kelly