When you turn your auto reply on, to run in the future, it’s in a “waiting mode”

– the button right to the auto reply changes its color

– outline arrow icon appears in the notification bar

auto reply in waiting mode

When you turn auto reply on, to run right now, or it turns into the “active / running mode”

– background of the auto reply changes to light blue

– filled arrow icon appears in the notification bar

SMS Auto Reply is active

the phone is silent (if not set different)