Hopefully someone can reveal a better way to do this in Typst, but for programming languages in general my favourite hacky approach is to check for equivalence between the string converted to lowercase and the string converted to uppercase. There will be equivalence only when there are no letters.

This only works for written languages that have the concept of upper and lower case, but that covers the countries mentioned.

It seems that two content blocks are always distinct, so I first need to convert the content to a string. But that does indeed work.

#let postcode(it) = {
  let s = it.text
  let l = lower(s)
  if l == s {
    it
  } else {
    set text(number-type: "old-style")
    smallcaps(l)
  }
}

This will work if the input postcode always contains upper case letters. If you need the function to be able to handle input postcodes with lower case letters (rather than rejecting them as incorrect), you could use something like if upper(s) == lower(s).

Thanks. You're right. I've made that tweak now. And given the user options.

#let postcode_set(it, apply: "alphanumeric") = {
  if apply == none {
    upper(it)
  } else {
    let s = it.text
    let l = lower(s)
    let u = upper(s)
    if (l == u) and (apply == "alphanumeric") {
      it
    } else {
      set text(number-type: "old-style")
      smallcaps(l)
    }
  }
}

// Use one of these options:
#let postcode = postcode_set.with(apply: "all");
#let postcode = postcode_set;
#let postcode = postcode_set.with(apply: none)