There are several ways to tell if an unlabeled bell is a Blum bell. One is to look at the shape of the bell. You'll notice in the picture that the bell does not have sharply creased edges that define the sides, but that it curves. This is because of the shape of the "former" mentioned on the red Holstein Bell label--"Die and Former for Shaping Patented." As you will see also, because of the curved sides, the bottom opening is oval and the bell flares out slightly around the opening. In the letter from the satisfied customer (see the link to "Customer Letter"), the farmer/rancher comments on how clearly his Blum bell could be heard across the lot. That is due to the shape of the bell, in addition to the brazing process.
Another feature to look for is the three-cornered fold at each side of the top of the bell. On a Blum bell, the corner was cut off with a definite point. I have seen other bells which had a blunt end to the fold. Also, the rivets that held the bell together down the sides were quite flat, which maintained the smooth and graceful appearance of the bell.
The loop through which the strap for the cow's neck would pass is flatter than loops on other cowbells I have seen pictured on eBay.
The 99%-of-the-time definitive way to determine if a bell is a Blum bell is to look up inside the bell. Please look at the bell interior pictured as you read this. Hold the bell sideways, as shown. You will see the small ring in the center top, from which the clapper was hung. (On the outside at the center top of the bell you will see a rivet that holds that small ring in place.)
You're looking for the ends of the large loop for the strap mentioned above, which were inserted through slits for that purpose and then bent inside the bell to keep them from slipping out again. The brazing process covered every possible chink around the large loop so that you can't see, from the outside or the inside, any evidence of the insertion of the ends of the loop.
HERE'S THE IMPORTANT PART -- on a Blum bell, the ends of the large loop were cut in points, not left blunt. One cut across the strip of metal that would form the large loop would do it. That cut was on a slant across the strip, not straight across. This is what you are looking for when you look at the ends of the large loop.
I have seen bells which were manufactured with the same process for attaching the large loop--probably this was a standard way of doing it--but only Blum bells had the metal strip cut this way. I have sometimes seen what appear to be blunt ends on a bell that otherwise looks like one of "ours", but only if the bell was in poor condition, very rusty, bent, etc., where the ends of that loop were already hopelessly misshapen or rusted away. In those cases, it really isn't possible to be completely sure the bell you have is a Blum bell.