When the subscription is expired, the name displayed in the top left corner of your own post should turn from orange to brown. Even though the difference is rather a bit subtle, chances should be that you notice that your subscription is expired.
I would be willing to pay more for the subscription., even though the value of JPY has been falling down drastically. However, I basically try to keep the subscriptions with the automatic renewal as little as possible, because the subscription will "stop automatically" when something happens to me.