You have to look beyond the States.
Flagship phones by companies that are international are phones that are sold everywhere.
The biggest red flag that the Infuse was NOT a flagship phone was that it was sold only in the U.S. and Canada. No such model exists in Samsung's motherland (Korea) or anywhere else.
Plus, if you are about to buy a phone, I would make sure you screen through tech sites like Engadget. When the Galaxy SII came out (in Korea and UK), literally every single tech site was raving on how this is the best phone of 2011 and how it is Samsung's flagship. I guarantee there was no such parade for the Infuse, other than "It was a gorgeous Super AMOLED+ screen!".
Think about it. Samsung has its market on a global domination level. Believe it or not, the U.S. is not the only country in the world where mobile tech market share matters (Korea, Japan, China, UK, etc.).
So you know there's something off when Samsung decides to release a single core phone in the States only (along with Canada), MONTHS after the Galaxy SII was released. That there should have been the second red flag. Why, when Samsung (along with HTC) set the new bar as their prime phones having dual-cores with 1 GB rams, would a phone be released in the U.S. 3 months later with a single core and half the RAM size? That was definitely fishy (a lot of people were saying the Galaxy SII could have been released in the States a lot earlier but Samsung was having a tough time negotiating with AT&T and them carrier friends).
So actually, it's not hard to spot a flagship phone.
The original Samsung Galaxy (AT&T's version was the Captivate) was a flagship because it (despite the American adjustments) was an international phone. That phone (again, in slightly different form factors) was released as a global project by Samsung. Same goes for the Galaxy SII (despite the utterly pathetic delay of 6+ months it took for the U.S. carriers to finally agree on selling it), and the same for the Galaxy Nexus.
So next time you want to buy a flagship phone, you have to look beyond just what's the mobile lineup in the States. These companies are all international, so you know that unless they're selling that phone and promoting it in all of their major markets, it's not a flagship. I mean, that's what a flagship is supposed to be anyways. It wouldn't be a flagship phone if it was only sold to one of their markets (and on top of that, the # of Infuse owners is tiny compared to owners for the Captivate, Galaxy SII, etc. even in just the U.S.).