first thing I look for is sitting on the cool end of the cage/tub this is a good sign of developing especially if its a constant thing where you hardly ever see them on the warm side.

then its bowl wrapping / sitting in weird positions sometimes my females will lay upside-down sometime kind of twisted on their sides this is a decent sign of developing.

then you look for the building. my females seem to just keep getting fatter by the day when ovulating is close. the bottom third of the body will have a lump like she ate a big rat that's when they ovulate. my females seem to go back to the warm side before ovulating.
after they ovulate they will sit on the warm side most of the time. often in a perfect coil.
the coil seems to get tighter the closer to laying
right before they lay eggs they become very active and sometimes twist and contort their bodies to move the eggs into position.