How does one differentiate between an exoplanet orbiting in front of a distant sun and a large starspot traveling across that star’s disk?

The light curve of an exoplanet transiting its star (top) is different from that of a starspot traversing a star’s disk, so astronomers can tell the two signals apart. Astronomy: Roen Kelly

Thus we can distinguish a transit by the short duration of the dip. … so the dip of a transit is slightly “U-shaped.” The shape of the starspot dip has gentle sides and looks more “V-shaped.”