Unfortunately, payment processors give us just as little information as they do to you when a purchase declines. Everything needs to be accurate - even the billing address needs to exactly match what's on file with your bank!
In the case of membership subscriptions, the automatic retry occurs days later, so it can be difficult to tell if you've "fixed" your card info as it's not charged immediately. In this case, one trick you can do is use our older Donation form to process a donation (can be a tiny amount) and the transaction will be attempted immediately, giving you immediate feedback on whether the card worked.