In theory it shouldn't matter which ID you access an item with. Can you elaborate what you need to know it for?

To find creator templates that point to a redirect so we can keep the data consistent.

Workaround: mw.wikibase.getEntity() always returns the target entity with the new id. So you can do:

local id = ...
local entity = mw.wikibase.getEntity(id)
if id ~= entity.id then
    it's redirected
end

I verified that the solution proposed at T68974 for regular redirects also works with redirect items (and there's no reason it shouldn't)

Oddly enough, the id of that task is an anagram of this one's. I'd like to know how many times such a thing has happened.

In T96478#1961396, @matej_suchanek wrote:

Workaround: mw.wikibase.getEntity() always returns the target entity with the new id. So you can do:

local id = ...
local entity = mw.wikibase.getEntity(id)
if id ~= entity.id then
    it's redirected
end

Matej, workaround seems to solve the problem. Maybe we should document the way to check for redirects and close this task, or do we want to add short trivial function to mw.wikibase to do the test?

Opps the task is already closed after code change. If there is such a function than it should be documented at https://www.mediawiki.org/wiki/Extension:Wikibase_Client/Lua