One indication that your choke is effective is when your opponent taps. An even bigger indication is when it puts your opponent to sleep.
The latter is what happened when brown belt Steven Robertson took to the Fight 2 Win stage a couple weeks ago at F2W 109. Robertson set up a beautiful loop choke on his opponent, and in just seconds, his opponent went limp, and the ref called the match.
Later on, Robertson demonstrated the choke for Fight 2 Win fans, and you can watch and learn it here: