Inside info is ALWAYS the best guide you can get, there is nothing like hearing from a reliable source that a horse has improved 3 lengths on its last run/spell, you can afford to go big knowing your a 90% chance of getting money back or more. When you can't get it I look at the following:
In my experience from being involed in horses/having family members heavily involved with various trainers, trainers are the worse judge of their own horses going around.
I've had it all from one prominent trainer stating in an email that "your horse will win" and having it run last, to being told that your horse is not a betting proposition and having it bolt in by 6 lengths.
The only thing i take from hearing an trainer talk up a horse is that it is fit and well. Other than that, take it with a grain of salt and do you're own form assessment.