How Do You Know if You Are in C Major?

We have a C major chord coming on the sub-median as we've seen. And then on the B, the seventh degree, we have a G major chord which I haven't been playing previously. So, these notes could only belong to C major or G major. They don't fit with any other key. It's very common for chords to alternate between the tonic and the dominant. This keeps a kind of movement underscoring the music.