Its kinda hard to tell with cresteds. Good signs are:
Slight weight gain that stays constant for a while (measuring it every few days will give you a good idea of normal weight and gravid weight)
A slight visual weight gain around the waste area.
I normally go by acctual weight though, this seems the best method for me. You can allways just leave the nest box in there all the time and watch for the female diggin (that's another thing I look for, it's pretty obvious as they come out earlier at night and stay up later in the morning.)