One way is to examine their respective views of what it is to be human - what it is, in the metaphysical sense, to be man. What follows is not true of all conservatives nor of all liberals, but I think it's fair to say that it is true of a great many, perhaps the majority, of both.
Perhaps the most fundamental difference in the anthropology of conservatives and liberals is that conservatives tend to see man as bearing the image of God, possessing immortal souls, and loved by God. This is significant because from this starting point conservatives,
- See human rights as divinely ordained and based in the will of God, and thus objective and inalienable.
- See man as fallen from his original estate and prone to sin. Thus follows the conservative skepticism of governmental power and the need for institutional checks and balances.
- See history as both meaningful, because it is the outworking of a Divine plan, and replete with lessons for the present because human nature doesn't change much.
- See science as a fruitful means of making sense of the world because the world was created by a rational being and yields its secrets to rational inquiry.
- See morality as rooted in a personal, transcendent moral authority who promulgates an unchanging moral law to which each of us is held accountable.
On the other hand, many liberals tend to see man as the product of the blind, impersonal, random process of evolution. For many liberals, particularly secular liberals, which perhaps comprise the majority, God plays little to no role in either the creation of the world or in human affairs. From this starting point, then, liberals often,
- See human rights as the product of a consensus of enlightened thinkers.
- See man as basically good and malleable, and evolving toward ever greater capacities and perfections.
- See history as an indecipherable, meaningless flux of events about which we can know little and learn less, since humanity is constantly evolving and changing.
- See science as the only trustworthy source of knowledge and the pronouncements of scientists as authoritative, if not infallible.
- See morality as an arbitrary, relativistic set of arbitrary norms which have evolved to help us get along with each other. There are no objective moral absolutes and probably no accountability for how we live in this life.
These disparate worldviews have profound consequences. One's starting point largely determines where one winds up.
If, for instance, human rights are simply a human invention then they're grounded in little more than the will and whims of those in power. They're just words on paper. They have no objective existence and can be discarded or changed whenever someone has the power and desire to do so. Indeed, to accuse a government of violating the human rights of its citizens makes no sense if those rights are simply whatever the government decides they are.
Likewise, if human nature can be altered and molded then the temptation to use government to compel people to conform to the image decided upon by the elites becomes irresistable. Since there is no objective right to liberty the government can and should do whatever's necessary to create the utopian society. That, of course, leads to Orwellian dystopias.
Ideas have consequences and the bigger the idea the more far-reaching the consequences.