Okay...this is how I ALWAYS do it:
1. Go to Amazon.com--either the Instant Video Store, or the MP3 store
2. Select a video or MP3
3. Look for where it says "Have a Promotion Code"? (in the middle under the 1st box or under the click to buy button, usually"
4. Next to that, there is a link to check your balance. Click that link. It will show your credit in each of 6 categories (i.e. Gift Card, MP3, video, apps, etc.)
I know Chris has posted another "check your balance" link in the past, but the above is A LOT easier for me to remember, so it's how I always do it. You can see how your gift card balances have been applied under "My Account". It will itemize exactly what has been deducted for what. (For example, turned my $50 camel win into an Amazon GC. Under my account, it shows $4 for a cable, $20 for some grocery items, etc.)
I rented Dark Shadows last night. My credit balance under Instant Video showed $4, but only $2 was applied to my rental. THe other $2 balance still shows.
Not sure how to check expiration dates. I can tell you this, though, If they apply the wrong balance, just contact them, and they will correct it. Had that happen to me during the Mp3 for a nickle promo. They deducted the full cost of the Mp3 from my Mp3 balance. I asked them to deduct the nickel from my GC balance, and refund the other 1.24 to my MP3 balance, and they did. So it always pays to check your balances before purchase, then check your e-mailed receipt to see if the balance was properly deducted.
Hope that helps!