How are we to discern the difference in which Old Testament laws need to be obeyed?

It’s not that some laws are to be obeyed and some are not. The principles underlying all of them are universal and therefore are to be obeyed. The problem is that we don’t know for sure what some of those laws refer to. For example, the mixing of fabrics may have been forbidden because the priest’s garments were of mixed fabrics, so to keep the priesthood separate (meaning of “holy”), the people did not wear garments of mixed fabrics. Or, it may be that merchants mixed cheaper fabrics with the more expensive ones, producing inferior products while charging too much for them. The underlying law still applies: Don’t cheat folks! Whatever the case, it’s certain that God doesn’t have some kind of issue with what kinds of fabrics our clothing is made of, just as he doesn’t insist on taking one’s shoes off when in His presence (unless that’s the custom showing respect and honor within a particular culture).

Tattoos, cuttings in the flesh, and cutting off the edges of the hair and beard were apparently associated with pagan religious practices at the time, so the underlying law, which is still in place, is: Don’t attempt to serve God the same way the heathen worship their false gods.

It is probable that some of those practices were magical rites designed to bring good luck. God is simply telling Israel to rely on Him and avoid putting your trust in magic and superstition for healthy crops, babies, etc. Archaeologists have uncovered evidence for some of these things, but there are still uncertainties about the specifics.

When I was growing up, the only people I knew who had tattoos were people who were in and out of jail or had been in the military. It seems that a good many of them got their tattoos when they were drunk, and they later regretted it. I suppose that’s why I was so disappointed when I discovered that my young niece had tattoos up and down her arms. Because of my background, I can’t associate tattoos with anything positive. However, I realize that the people who get them today usually do so for reasons other than magical or religious or as a sign of servitude, so I can’t tell them they are in violation of God’s law. The best I can do is tell them that I think their tattoos are ugly and that someday they will most likely regret having got them.

All we can do is do our best at discerning the purpose of any given law and then ask ourselves whether and how that purpose applies to us. Here is an example: Moses commands that a parapet be put on the roof of one’s house. The purpose of that law was the protection of the people who would go up on the roof and use it as a living area. Have I disobeyed that law by not putting a parapet around the roof of my house? No, of course not. My roof is not used as a living area; I’ve never had or served a single meal upon it. But I am to obey the underlying principle, which means that if I dig a swimming pool in my backyard, I must fence it in or do something to make sure my neighbor’s children don’t wander into it. It’s called loving one’s neighbor as oneself. Here’s another example: I recently underwent surgery—the surgeon cut into my flesh, and I have the scar to prove it. Now the law forbids cuttings in the flesh. So did I violate the law? No. The Bible does not forbid cutting the flesh for the purpose of fixing a problem. It does forbid cutting the flesh for the purpose of getting a “god” to do a favor for you. In one way or another, all those laws that seem odd to us had some kind of purpose like that—either for the protection of human life or the preservation of God’s way of life as opposed to heathen religious/magical practices.

I think you can see why I cannot honestly say that the law forbids tattoos for any and all purposes. I don’t have to like tattoos, but I would not feel right about telling someone that getting one is in itself a violation of God’s law.