The only way I believe you can tell is that the female is fatter. I have a spiney eel that I am convinced is a female because it is about 7 inches long about an inch in diameter. She is fat!!!! I just bought another spiney eel which is in quarantine at the moment. It is only 3 inches long at the moment but it is much skinnier then Squiggles, my first eel, was when I got her. I am hoping it is a male but it may just be skinny because they are not fed well at the store.
I would be extra careful when adding new fish to the tank because eels don't have scales. This makes them extra sensitive and susceptible to illnesses such as ich. Make sure you quarantine all of your fish before adding them to your main tank. This will also cut down on the amount of water that you will need to treat in the future.