A practice throw is defined in the rules thusly: "A throw of more than two meters during a competitive round that does not change the lie. Provisional throws, misplayed throws, and stance violations are not practice throws."
So, by definition, a throw made from one's lie that is intended to progress the player down the fairway can't be a practice throw. It isn't when it's thrown and it certainly can't be declared one retroactively.
Tough to say what is an "acceptable" practice throw since, by rule, it's never actually acceptable (thus the penalty). An example of what is a practice throw would be if one were to miss a putt and then pick up another disc and throw the putt again. The second throw would be considered a practice throw, it wouldn't count as a legal throw (so it couldn't be used to determine the next lie), and a penalty would be added to the player's score. Another example would be in the event of a back-up at a tee, you're not allowed to go back to the previous basket and putt, or play catch, or toss discs in a nearby field to "stay loose" during the wait. Those would all be considered practice throws.