> Is anyone but you conscious?

Yes.

> If so, how can you be certain?

I can't be absolutely certain of that nor of anything else. The philosophical-zombie hypothesis cannot be ruled out by evidence. But I observe others producing complex I/O behavior that is qualitatively similar to that which I produce and that I attribute to my own consciousness. I also see evidence that my consciousness supervenes on the operation of my brain, and I observe that other people have brains. The most parsimonious explanation for all that is that other humans are also conscious.