Edit to add - I can't figure out how to upload the pics. Can someone help?
- And got the T-shirt
Can you feel the penis bone on the one that you're not sure about? Put your index finger crosswise of the abdomen, just above the genitalia, and press down firmly but not too hard. Move your finger back and forth, and if there's a bone underneath, you should be able to feel it rolling under your finger. You can practice on the one you know is male.
There are some more hints here: http://www.cavyspirit.com/sexing.htm and here: http://www.guinealynx.info/sexing_pups.html