To explain my stance vaguely, if you can make critical decisions about abstract concepts, then I'd consider you senior. For instance, I recently saw that some of my coworkers were using an HTML tag in a way that was technically correct, but in a situation where a different tag is more suitable based on the context of the page. This is something I've learned through my studies and experiences as a front-end dev, and as such, contributes in a small way to my "senior-ness".
> If you can quantify how you're the "go-to" guy
Pretty much when anyone has front-end problems they can't figure out (either by themselves or even after Googling), I'm the one they come grab. Mostly "Why won't this div go where I want it to go?" or "How do I make this stop overlapping that?" - or my favorite, "Can you help me make this work in IE8?". Not to portray my coworkers as whiny, btw. I think they just want to be done with the front-end stuff as quickly as possible, so they can get back to the back-end.
You did say quantify, so I'll say I probably provide front-end help to a coworker at least 4 times a week. One coworker maintaining the last project I worked on consistently expresses his gratitude for laying the foundation that I did on that project (front and backend). This is starting to sound like I'm bragging (trying hard not to make it sound that way), so I'll stop here.