How do Catholics know which OT laws are moral and which are merely civil or ritual?

I understand that Jesus fulfils the OT ritual laws and one's country's laws replaces the OT civil laws, leaving Catholics with only the OT moral laws plus the 10 commandments. How do Catholics determine whether a commandment is a moral law and binding? For example, ploughing with an ox and an ass in the same yoke (Deut 22:10). Is this some spiritual purity law or is this about compassion to the animals (for the ox will push harder than the ass and drag it along)? What of laws with *both* ritual *and* moral components, such as the Sabbatical year which frees slaves and annuls debts as well as forbids planting? Is there a rule or did they just discuss it in a synod?