Searching between two separate lists for matches

I have two SharePoint Lists each with a column called Unique Key, what I need to do is search list B for matches in List A.

List B is the larger of the two and each List contains only unique entries in their respective lists (with matches between the two lists)

A separate column will be populated in List A stating if a match has been found with all values in List B.

Is this possible with OOTB functionality?

I could move List B's contains to List A if that makes getting a solution easier but I would prefer not to.