how would one distinguish an objective morality from a subjective one?
That’s actually a mighty fine question.
There are plenty of people out there who mistake their personal beliefs and feelings for “the way things are” in a universal, mind-independent sense; and then seek to impose those beliefs and feelings upon others who don’t share them - often by ascribing their personal feelings to their deity-of-choice.
Certainly, there are things that are objectively harmful and objectively beneficial to living beings, including us. But morality is about finding reasons to do the things that are beneficial and avoid the things that are harmful, in a general sense.
I’ll try to illustrate with the simple example of a decision over whether or not to eat chicken. There’s no doubt that it’s objectively beneficial to me to eat chicken, from a nutritional perspective. But it’s also, undeniably, objectively harmful to the chicken! The factors involved, then, in my decision over whether to eat a chicken have to weigh the benefit to myself against the harm to the chicken, and I think this can only really be done from a subjective point of view.
If I don’t care, one way or the other, about chicken welfare, I will just eat chicken because it’s cheap and tasty (although even the tasty part is getting into the realm of subjective feelings). If I believe human life totally trumps all other life, I might similarly disregard the welfare of the chicken and eat it anyway because it will benefit me (and anyone else for whom I might be catering).
If, on the other hand, I believe the chicken’s life is of comparable value to my own, I would probably avoid eating it on that basis, and seek other, more humane, sources of nutrition in which no suffering is involved on the part of any sentient creature. For many people in Western nations, this is a readily available choice these days, and I’ve met plenty of people who’ve made it.
If I think that the chicken’s welfare matters while it is alive, but also believe it is perfectly natural (and also objectively beneficial) to consume meat, this will also affect my decisions, in the form of compelling me to buy chickens that have been raised in a humane, free-range fashion, and probably less often than I would if I were just unthinkingly guzzling factory-raised birds. There are objective considerations involved here also - it’s more expensive (but also more nutritionally sound) to consume free-range birds - but ultimately, the decision comes down to subjective preferences.
Some religious believers claim that the will of their god provides the basis for ‘objective’ morality, but I’m not sure how the preferences of a personal god are any less subjective than the preferences of a human person, or any other sentient creature. Some would also claim that without a god standing over us, or without the promise of eternal reward or the threat of eternal punishment, we would not feel compelled to act morally - but this is to misunderstand the power of subjective preference. Yes, of course there are objective facts involved in our decision-making. But I think it is the consideration of the subjective preferences of all interested parties - the moral calculus - that sets moral decisions apart from mere practical considerations. The only difference, for religious believers, is that they take into account the subjective preferences of their chosen deity, not just their fellow mortal beings.