The first indicator that they are developing is that they 'chalk over' - meaning they become less translucent. Folks recommend not handling the eggs at all, just letting them be. That way the embryo doesn't get knocked around, possibly hurting it or killing it. You can very carefully candle the egg without touching it with an LED pen light. Nothing much to see until about 4 or so weeks, at which point little red veins become visible in a fertile egg.