From a research point of view, they would screen those people out early or would be able to tell from the data that something is amiss and exclude it (assuming a person who was not attracted to other humans was included in the "control" group).

As for identifying zoophiles, I believe PPG has been used successfully for that group, although I'm not aware of much furry research given that most furries are attracted to humans, even if they prefer to date within the community. Furries are also not typically a forensic group and most don't need treatment, so there's not a lot of money invested in figuring out what's going on in male furries' pants. It would also be a slightly different situation because having sexual arousal when you imagine yourself as something that you're not (for example, imaging yourself as a fox) is not quite the same as lacking sexual arousal to other people. To put it in a more common scenario, someone with body image issues (say, they are overweight or embarrassed about their body) may be capable of sexual arousal but their mind would stop themselves from getting into sexual situation in a real world situation.

It's all kind of a big, beautiful, complicated landscape of arousal reactions (which I am only familiar with in passing, I should warn that this is not my main research interest as I'm more into normative female sexual responses and aging).