For me, it's more along the lines of having business manners?
When you have it, people don't notice. But when you don't have it, people WILL notice.
So it's more noticeable for me when I see someone without sincere passion. That's mostly because I understand not everyone expresses their passion outwardly and vocally. Some of us are megaphone excited and it's in your face. Then there are some of us who are more quieter about it and prefer to express that passion in more private settings. Honestly, a person could have zero internet personality on their profile (just posting their art and providing information), and I still wouldn't say they're not necessarily dispassionate.
But the ones that bring me doubt are the ones that A) constantly talk about how they hate their projects, B) constantly talk about how they're not getting any numbers and views coming in and C) have little to nothing nice to say about their work. I can understand still being passionate about your work and also doing these things. Especially if you're getting paid to do this comic -- a lot of us do get jobs we might not like. But often enough, I see this coming from people who don't have this comic as a source of income; they're posting online for free. It just makes me believe this someone isn't really happy with what they're doing but doesn't want to admit it right away.
The sincerity is less about being this vocal cheerleader and more about...having clear conviction to your work, if that makes sense. How you express your sincerity for your work can definitely differ. But when I see nothing but negative vibes from a creator about their series and they're still trying to tell me they're "happy to work on it"...I kinda press X to doubt. There's only so much you can do and how long you can last to fake sincerity. At that point, you're better off admitting you're not into what you're making: at least you'd be honest with yourself and your audience.