> 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.
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".