Usually they gain quite a bit of weight- 100-200 grams- in a relatively short period of time. You can also _sometimes_ palpate the eggs. This ability has a lot to do with experience in getting the snake to cooperate and relax enough to let you feel them. I let the snake crawl on the floor, between my thumb and two fingers, but the snake has to be going in a straight line, and has to be relaxed enough for you to feel bumps.
If you have bred your snake, and are reasonably expecting eggs, I personally would put the lay box in with her, and replace her large water bowl with a small bowl, as soon as you are done breeding her. They usually lay 8-14 days after the shed after breeding, but sometimes lay before, and the viability of the eggs depends on being laid in an appropriate place. If the snake doesn't have a lay box, she will hide them somewhere, and they will desiccate before you find them.