Look for large big box stores that conspicuously have a product line absent, or only have a very defined subset of the product available.

For awhile iPhones were just completely missing from some larger stores, but it seems Apple has the upper hand there now and they're back in business. And even now Apple holds a very right reign on the prices charged (Best Buy seems to have the most leverage here, as they often sell new MacBooks for a bit lower than the Apple Store). This is one of the reasons Apple started the stores; gives them leverage against the retail giants.

There are a number of brands you'll never see at Costco; they don't play the game.

Hmm how do you differentiate between supply chain shortages and this scenario?

Supply chain shortages means you used to see an item, but it is not available at this time. Vendor arrangements means you've never seen Del Monte can corn at Costco, but you've seen Kirkland can corn that tastes just like Del Monte's.