Often you can tell by the shape of the neck. The Conn straight neck is a dead give away... Duh...
When it comes to tenors, let me grab my C mel Handcraft off the wall, and my similar era tenor out of its case. I'll photograph side by side for you. The necks in some brands have a squatter look to them on the C mel models.
Now to totally confuse things, take a look at these 2 horns. The larger one is my Series III Toneking from 1957, while the other is a C-pitched tenor that I didn't buy when I had the chance (idiot probably) that Adolphe Edouard Sax built.
The C pitched tenor is currently getting restored at my tech's shop. Once it's done I will be recording a video with it, and then... Who knows... I might buy it yet... Or it will continue to sit in its case in Vancouver with a non-sax player. Or he'll decide to sell it for way more money than I'm willing to pay...