Welcome to the AA boards...

As for checking availability of sticker upgrade seats, there is no way really. AA doesnt hold a specific inventory for sticker upgrades. That I know of they only hold X amount for mileage awards (full award or upgrade award). Sticker upgrades are based on availability of space on the flight between 100, 72, and 24 hours of the flight. Thats the time limit they have for requests. As a PLT you have 72 hours to request an upgrade. You can put the request in once you book your flight, but you wont be confirmed until 72 hours (or less) before. The only way you wont be able to get an upgrade is if all FC seats are sold before your time period, EXP's take up the sticker upgrade seats, or if other PLT's are on the list for an UG before you are.
What I have done in the past was look on the AA website on the seat maps which show availability of seating in a flight. If i see plenty of seats available, based on my experience, you may get the UG. All depends on the route you fly, time, day, etc.

Hope this helps.

-Eli

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Washington,DC/Miami
AAdvantage Platinum