Unlike pets and mounts, you can't actually look up which tabard you've worn on the WoW Armory. Is there a way to tell? My memory doesn't always serve me the way it's supposed to.
No - unfortunately there isn't any method of tracking the individual tabards you have worn. I generally keep all of mine stored in my bank as a method of keeping track of which ones I have worn to date but if you've gotten rid of any of your tabards then you're just going to have to work through them all until your progress counter increases.
Don't forget that you can easily reclaim any tabards that you have destroyed by using the tabard vendors in your faction city, Dalaran and Shattrath.
Your best bet may be to download a tracking addon such as Tabard Addict.
This comment on Wowhead's Thirty Tabards achievement page suggests that the following macro may be able to give you a complete list. However, another comment says this does not work post-5.0.4. (I have not had the chance to verify.) See the comment for further details, it may require some editing.
/run y=GetAchievementCriteriaInfo for z=1,87 do a,b,c,d,e,f,g,h,i,j = y(1021,z); k,l,m,n,o,p,q,r,s,t = y(j) if m==false then print("\124cffff0000\You have NOT YET equipped:\124r",k) else print("\124cff00ff00\You have equipped:\124r",k) end; end