I'll noticed a little, then the will burst out with a lot pretty quick. I do notice subtle indications before...usually. They may be mating already, mate again later, and maybe get a quickie before you shut the lights out too.
I think ones chances of seeing "it" happen are better when the males are kept separately, then females introduced. If you see any headbobbing, or push-ups, chances are he's already done his job. She can be gravid for 2-4 weeks. I think its related to how much she is, her overall health, conditions of the tank, etc.
Incubation time can be anywhere from 40's - 50 some days. Check some reptile classifieds...ask around about incubators. Or just keep them in a decent temp room. They may take a bit longer to hatch, but I think they will be fine anywhere from 75-82.