How to check if a tag exists in a measurement using flux?

simple you need to add a filter for the tag name you need to filter to your flux query. let's say the tag name is "tag_name":

|> filter(fn: (r) => exists r.tag_name)

this will filter any undefined or "unset" tag.

(answering for my usecase, since no details were provided)

TL;DR: use the exists operator.

Imagine a dataset which has this old tag, which we don't care about anymore, and we want to filter out those records when archiving:

option task = {
  name: "15min_archive",
  every: 1w,
}

from(bucket: "mybucket")
 |> range(start: - duration(v: int(v: task.every) * 2))
// this:
 |> filter(fn: (r) => (exists r["stupidOldTag"] ) == false )
 |> aggregateWindow(every: 15m, fn: mean)
// ...

note: the ==false because not is apparently not a thing yet.