Sometimes you can't tell. There is no gestation colouring. Non-receptivity isn't an accurate measure - she may simply be non-receptive. With some Jackson's the female may show initial agression towards the male when receptive.
Some chams will appear larger and non be gravid. Some gravid girls may fool you right up until the day they deliver. Masters of disguise these girls! I'm in the same boat - have a female I think is gravid. She's gaining weight, eats like a pig, basks intensely - and gets along really well with the male. ???? If she's gravid she'll be ready sometime in June - I'll just have the fruit flies and pinheads ready!!
A good sonogram or x-ray may give you an idea..
She may initially increase in appetite - then start to cut back 4 weeks before delivery. Her agression towards males may increase.
On the day of delivery she will stay on the lower branches and give birth in the late morning. Watch the cycle of the full moon!
Not much help am I?
Jaylyn