Do hunters have to be sure that the thing they were dealing with wasn't human to get practical experience? Could they get experience if they only suspected the thing wasn't human? As a sort of aside, do they get practical experience for mortals, such as slashers or mafia members?
I've been mastering Hunter: The Vigil for a while so here's my take on the matter :
It entirely depends on the situation.
What I do, usually, is give them PE whenever they have an encounter while hunting. May it be supernatural or not. The important thing is to remember your player characters are fighting for the Vigil, and it's hard. It's gruesome. Supernatural or not, they deserve whatever experience you can give them when they come out alive of those encounters.
Another important point is to make the PE necessary. The players must feel they need those points for future hunts, that it will actually be useful.
That's my two cents, anyways :-)
The books themselves don't make the distinction of whether or not your Hunters (or Players) have to know for sure to get Practical Experience from a monster kill. A good general guideline is to ask yourself (or have your players justify) whether their actions are part of their perceived duties as a hunter, "keeping The Vigil."
If they had reasonable suspicion, came up with a plan, and executed it properly, that's a good case for awarding PE. If they, for example, accidentally struck and killed someone with their car who also happened to be a rogue Mage that's a good case for not awarding any.
The guideline is also good for handling non-supernatural kills. If your Hunter cell are general vigilantes, then certainly taking out gangsters and serial killers could count. They're picking up knowledge of how to take out these same targets better in the future. If they focus only on supernatural threats, killing or otherwise getting rid of mortals who may be protecting or assisting them would be part of "keeping The Vigil," while general crime fighting could be considered a matter best left to the police.
Tier-2 and Tier-3 Hunters will be part of organizations with codes of conduct, stated goals, and other structure that can also help you better define what does and doesn't count.
Practical Experience as written is the reward for learning from combat. Let your players prove what they learned from a given combat, and you can justify giving that XP. For most hunters, combat isn't a way of life, and fighting against Mafia members would be a totally new experience for them, something they can learn from.
The flip side to that argument is they might have less in total they can learn from fighting mobsters than fighting supernaturals, which might be reflected two ways. You can either give half the XP for it, or give them full and really challenge them to learn something from it to continue gaining experience.
As an example, Ashley and her fellow hunters are engaged in combat with several Mafia members, one of whom is armed with a submachinegun and the rest with handguns. They take cover behind some walls. If they hadn't used much of cover in combat, then you can use that as something they have learned, that cover is an effective defense against firearms. The mobster with the automatic then lays down a suppressive fire on the area where the party is hiding. If the party has never dealt with that, they can use that as something they have learned. Finally, the barrage of bullets end and the guy with the automatic can't shoot for a round because he expended all of his ammo. The party has learned that suppressive fire and/or long bursts chew through even large magazines quickly.
Thus, practical XP can be obtained through combat with mortals because they are still learning from the experience. Once the party reaches a point where they have nothing to learn from fights with mortals, it will be much more difficult to get that experience through supernaturals anyway, as they are learning not from the combat itself, but from the supernatural modifiers.