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Wait for him/her to be flat against the terrarium wall and take a picture (as clear and zoomed in as possible) of the abdomen on the lungs side focusing on the part of the abdomen located between the first pair of lungs
Attaching a picture of my metallica as an example.
When I enlarge your picture, it looks like a male bulb to me. Maybe you can take a picture from underneath through the glass?
This is what I was looking at: