To find out which levels you have "completed" (destroyed all collectables on) and which you haven't, you'll need to find your save file.

You can find your save file in
%USERPROFILE%\Documents\My Games\BlueEstate\BEGame\SaveData\
(if you're using a Mac or other operating system this folder directory may be different).

Open Steam.sav with notepad or another text editor.
Use the find function (a shortcut is usually ctrl + F) to search for one of the following lines of text.


Each of these text lines is tied to collectables from one of the 7 levels in the same order. Each level has 8 collectables, for the achievement total of (7*8 equals) 56 collectables.

The Twin Dragon Part 1 --- Lucky Cats
The Twin Dragon Part 2 --- Lucky Cats
Bring Out the Dead --- Skullflowers ("CEM_Skull")
Tunnel of Murder --- Golden Chihuahua Statues ("UT_Collectable")
The Maltese Chicken --- OKFC Eggs
Golfing with Grenades --- Red Party Balloons
Bloodshot --- Blue Tribal Masks ("Jam_Collectable")

Your save file will have one of these text strings for every single collectable in the game. Additionally, when you destroy a collectable your save file will add another identical text string for that collectable (but not always straight away, maybe after leaving the level). If you count one of these text strings less than 16 times in your save file, that means that level is incomplete.

Unfortunately the ordering of each collectable in your save file doesn't seem to match very well with the other in which they actually appear. E.g. 3_UT_Collectable_06 appears second out of the Tunnel of Murder collectables in the save file, but doesn't correspond with the second golden chihuahua statue that appears in the actual level.

TL;DR
Ctrl + F each of those text strings and count them. If you don't count sixteen of one text string, that level is incomplete and you should focus it with a video guide.