What is the best way to differentiate operant conditioning from classical conditioning?

I think...

With classical conditioning, the conditional stimulus is presented before the unconditional stimulus to form a conditioned response. For example, training a dog to return when a whistle is blown.

With operant conditioning, reinforcement is presented after the response. For example, rewards (positive reinforcement) are given for good behaviour. Punishment (negative reinforcement) for bad.

Hope this helps...

Classical conditioning was discovered accidentally by Pavlov.