Classification and discovery of stars How do astronomers find the difference between a 'Cephid Variable Star' and distinct normal stars with opaque objects blocking it's light, like stars with planets revolving around them while both create a dip in the brightness-time graph at regular intervals of time, when this is how new stars are discovered?
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.