This is not an exact science. I have some here of my own that I am not sure whether they are gravid or not. It is fairly common for them to breed and not become gravid. Things to look for:
1. Gravid females spend more than typical amount of time thermoregulating in odd, sometimes inverted positions.
2. Gravid females look lumpy and go off feed within a few weeks after males are no longer trying to mate with them.
3. Palpate, let the female crawl between your fingers appyling light presssure of fingertips up into body cavity. As the snake crawls through egg masses will feel like lumps. Sometimes you can even count them. Lumps are small early in gestation and grow to about the size of a hen's egg later. With really large females or females that tense their muscles this method does not work.
4. Hold the snake suspended from the forward part of the body. Gravid females will often look very lumpy along their belly scales in the aft third of body ahead of the vent.
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