Classification and discovery of stars
Cepheid variable stars and exoplanets transiting stars have very different light curves (the relation between brightness and time).

Exoplanet light curve from NASA:



Cepheid variable light curve from astronomynotes.com:



Also, as Rory Allsop points out, the scale of the change in brightness is very different. Cepheid variables can be seen at great distances, which allows us to use them to measure distances by their regular period-brighness relation, whereas exoplanets can only be found more locally by this method.