... Or you can just make an API call and find out for yourself:

Available: https://edit.yahoo.com/reg_json?AccountID=mister_popular@&Pa...

Unavailable: https://edit.yahoo.com/reg_json?AccountID=test@&PartnerName=...

Step 1: Create site offering 10-email watch list for $1.99

Step 2: ...

Step 3: Profit?