It is tricky often, but there is always a way.
For instance, cross-selling. I can use scanner data (what they record when you use Continente card, for example) to see the basket of products. For instance, I might realize that people always buy the same beer (brand B), but 50% buy chips (brand C), while the others buy just random chips. Then, if brands C and B belong to the same firm, they can create the "party pack" that includes the beer and the chips. Probably, most people will buy the pack. Therefore, the value of the chips brand C was not selling is the cross-selling.
In practice, it is usually better to look at M&As of similar firms and see the synergies they had.