How does a k-9 alert an officer when he has found drugs in a vehicle?

This depends greatly on the individual dog, the training given, and the relationship with the handler. Some will sit, sum will scratch or dig eagerly at the point of scent, some are "pointers", and some will bark. There are probably other signals, too.

Many "alerts" are very subtle, and so it's up to the officer to know his dog well and be able to recognize when the animal is alerting.

For testimony and probable cause purposes, courts generally accept that a certified handler is fully qualified to be the sole judge of when their animal has alerted to a person or object.