Hi Dennis,
Yes "he" is quite the character indeed!
There are ways of telling the sex, but not without handling the dragon, it is usually possible to determine the sex of a reptile by looking at its ventricle flap, males will have a slightly narrower opening than females.
They do have quite powerful jaws, and I can tell you that you really REALLY dont want to be bitten by an eastern water dragon past the age of 6 or 7 months. It will be rather excruciating. Its probably best that you just continue to assume its a female.. I'm quite sure he/she does not mind being called Lizzy!
My little dragons are still a bit young to determine sex by ventricle examination, however Bubbles is starting to show the personality characteristics of a female, where as Squeak is definitely acting more like a defensive male.. Time will tell.. As far as colours and markings are concerned, during summer the males will become brighter in their colour, and be quite intricately patterned, where as the females don't change much coming into breeding season.. They reach sexual maturity in about a year, and providing Lizzy has a good supply of food, basking areas and somewhere to swim, I dare say she'll still be around your house in a years time.. Then you should be able to tell...
Alex.