Hi Rachael,
STEREO mission is not particularly designed to detect sunspots. But the imaging instruments on board take pictures of the Sun in four different wavelengths, where solar active regions over sunspots can be seen as bright regions.
If you mean whether STEREO can see 'sunspots' that we can't see yet from Earth, then, yes, STEREO B spacecraft behind the Earth can see part of the Sun before we can see it from the Earth, due to the viewing angle.
STEREO satellites travel almost within the ecliptic plane, that is near the solar equator, and therefore are not designed to observe the solar polar region either. Also in general, sunspots appear below 50 degrees of solar latitudes, that is to say, we do not observe sunspots near the solar polar regions.
Yan