I don’t believe there is a way to see your combined [Insert Profession] level with the current crafting panel. There may be an addon that does it, but nothing springs to mind at the moment I’m afraid.
Otherwise you’re looking at good ol’ pen and paper or a calculator!
As far as I’m aware, the achievement just combines all of your existing knowledge of [Insert Profession] across ALL expansions into a total number to determine your profession ‘Level’ on any given character.
Once you have collected 525 Leatherworking skill points in total, the Cheevo should just pop.