Old Kent Road Post Office: How could I know item inside packaging with tracking number

I have deal with ebay user account and ok to buy item on his store,and he send me(Thailand) item at old kent your post office, and he gave me tracking number so I want to make sure he has sent me correct item that I bought

my question is How could I know what really is in the package that shiped with tracking number that he gave me?

thank you
regards

---