She stretches out, lets out a yawn, and fluffs her pillow.
Sorry.
Seriously, if this is a first-time for the hen in question, her comb will turn more and more bright red over time, and she'll start to exhibit the behavior like being in the nest box.
Are you asking for a first-time chicken, or are you asking when you can tell on a daily basis for a hen that's been laying for some time?