Certainly not, and you're right to ask. The short answer is that there isn't a well-justified standard and there can't be, until some really key problems in the philosophy of mind are first solved. We can rely on excuses like the language barrier between us and the animal kingdom to explain away our fallibility in trying to evaluate and interpret their mental contents, or the profound difference in cognitive and sensory hardware between us and them, as in the fantastic "What is it like to be a bat?" (http://www.consciousentities.com/bats.htm) but the much more uncomfortable truth is that we humans don't even know that each other experiences desires and pain and whatnot, except insofar as we take one another's word for it. The Vulcan question is pretty darned important here.

So what do we do in the meantime? We proceed according to the ad-hoc definitions we've already got, and go on identifying things as "experiences things" and "doesn't experience things" according to how much their behaviour resembles how we imagine we'd behave in their situation, and based on how effectively their body language communcates mental states that we think we can relate to, and eventually we'll find a hyperintelligent tar pit on a distant planet who teaches us* that our definition is incomplete. In the meantime, eat plants, but justify it behaviourally and not anatomically, because that at least has something to do with how we go about trying to infer the minds of others.

*by devouring hundreds of people