The vent gets bigger when they lay, and shrinks again when they stop laying (molting or broody or any other reason.)
The pelvic bones also move back and forth, when a hen is laying or not. So it can tell you whether she is laying now, but it can not tell you whether she has ever laid in the past.
I agree. Chickens typically don't lay eggs when they are molting. I don't know whether this would be the last molt as she grows up (because chicks molt several times as they grow), or it if means she's an older hen that is molting because it is fall.