How do I check whether the alias I paid for was successfully registered?  Is there a way to test it out?

Keep watching transactions in [My transactions] widget. U'll see that ur transaction is confirmed.

Got

{"errorCode":5,"errorDescription":"Unknown alias"}

What do I do now?

Come-from Beyond, could you maybe add a simple security measure to the Nxt account "login"?
Would it be possible to add a cool down time, so that when you open a account you have to wait, lets say 5 sec befor you can open the next account?
That would help allot against brute forcing accounts....
I think that is something wary important !

It's impossible in decentralized system.

It's fine. U sent a transaction but it was rejected by other peers. Alias System will be activated at height 22000.

Will I have to do it again after block 22000?

Are there any alias that are reserved, preallocated or taken already?

Updated nxtbase8, nxtbase9, and nxtbase10 with experimental.

Great, let me know if u see any issues, plz.