Hi Mojo,
In the majority of cases a dog handler will be honest about the limitations of their dog. Mainly because this is their baby so they don't want to overstretch the dog (not like a foot searcher who will go on too far!). I know that I am guilty of underestimating what my dog can do but probably as he is still in training.
As far as alerts go it can be anything to be honest. Guinness (Daryl's dog) will run at us head down tail straight at a million miles an hour, nearly knock you off your feet as he turns round and runs off at a million miles an hour. Scrumpy (my dog) either jumps up or grabs at my hand. Some other dogs sit at the handler, grab at a toy the handler has or barks at the handler (not the MISPER). Basically in English the dog can do anything that they don't normally do during searching. As the dog gets more experienced they adapt their alert as they haven't got time to do the alert they used to. For example Guinness used to jump up initially, this then changed to running right up to your feet and turning round, now he runs at you and squeaks as he turns round and dashes off again.
So while the handler will train an alert the handler will just know by the dogs body language whether they are searching, on scent, or have found and are alerting. You just get to know your dogs habits.